Question 1

Should I split AGND and DGND in the MAX30001?

Accepted Answer

Its not advised to split AGND and DGND while designing a circuit with this device. Hence you should use a single ground plane under the device.

Question 2

What type of capacitors are recommended for use with the MAX30001 to minimize the noise?

Accepted Answer

Its recommended that you use C0G type ceramic capacitors in the signal path to reduce signal distortion

For the ECG path this includes CHPF, CDME, CCMEP, CCMEN 
 For the BioZ path this includes CDRVP, CDRVN, CDMB, CCMBP, CCMBN

Question 3

Can I miss to connect 1nF shunt-cap connected to CPLL of MAX30001 but directly connected CPLL to GND?

Accepted Answer

This is critical. Unfortunately, the user will need this 1nF cap to keep the PLL stable. It cannot be grounded. Please try to rework the board by adding 1nF bypassed to Ground.

Question 4

What are the leads of ECG?

Accepted Answer

ECG signal is a kind of differential signal measured on two different parts of the human body. Therefore, two electrodes are necessary to obtain one channel of ECG signal. Then, operators should place electrodes on two different parts of the human body and connect them to the positive and negative pole of the electrocardiograph through lead wires. This method of connection to record the electrocardiogram is called the lead of the electrocardiogram. An internationally accepted lead system is the standard 12-lead system, which has three channels of standard limb leads--I, II, and III, three channels of augmented unipolar limb leads--aVR, aVL, and aVF, and six channels of chest leads--V1 ~ V6. At this time, a total of 10 electrodes (one for left arm, one for right arm, one for left leg, one for right leg and six for chest) need to be placed on the surface of human body. Some of the electrodes can be reused, so it is not necessary to use 24 electrodes. Generally, in the case of normal use, one or more channels of the standard 12 leads are selected for measurement. Measuring one lead occupies one channel of the sensor.

Question 5

Why most Maxim's reference designs for ECG application have driving electrodes (Right leg drive)?

Accepted Answer

Because of powerline interference, the ECG signal contains many common-mode components. Sometimes, these common-mode components can affect the waveform seriously. To meet the requirements of improving the overall common mode rejection ratio of the system, the driving electrode is usually the output electrode, which output the common-mode component of the ECG signal collected by the ECG front end to the human body in the reverse direction. Generally, the driving electrode can be placed anywhere on the human body, but it is better to put it on the right leg, which is also called "right leg drive", because the distance from the right leg to other input electrodes is approximately equal.

Question 6

How do I ensure that the ECG signals couple the same common node noise?

Accepted Answer

Since ECG electrodes would not be run in parallel over their entire path from electrode to the module, one approach to ensure both ECG inputs “see” the same noise is to add an antenna that runs in parallel to the other signal. This technique ensures that the external interference (RF noise, for example) remains common-mode to both inputs. If the environmental noise is common and in phase to both ECG inputs, it will be attenuated by the instrumentation amplifier thereby enhancing the performance.

Question 7

What is the maximum impedance the MAX30001 can measure?

Accepted Answer

The maximum impedance is limited by the input voltage range of the ADC and the drive current. With the minimum PGA gain of 10 V/V, the lower drive current of 8uA, and the ADC's input voltage range of 1V, the maximum measureable impedance is 12.5 kOhms. To measure larger impedances, a resistor can be placed in parallel with the measured impedance such that the parallel combination is less than 12.5 kOhms. After measuring the impedance, the larger impedance can be back-calculated.

Question 8

When will the MAX30001CTI+ (28 TQFN pkg) be available?

Accepted Answer

Answer: Maxim currently supports only the WLP package type and we have no immediate plan to offer the TQFP package type until there is sufficient market interest in that package type to do so.

Question 9

How do I optimize Maxim ECG AFE registers?

Accepted Answer

Maxim ECG AFE (MAX30001) is a complete biopotential and bio-impedance analog front end solution for wearable consumer products. In addition to the AFE amplifier and ADC commonly found in competitive parts, the MAX30001 has a built-in sophisticated ECG detection algorithm that minimizes software development time for the customer. Once proper register values are configured, the user can read heart rates detected by the built-in algorithm over SPI. For those who want to plot ECG over the screen, raw ECG data is also accessible through SPI from buffer storage. 
 The built-in algorithm is based on a differentiator-integrator algorithm modified from the popular Pan-Tompkins solution. Since the differentiator is used in the first phase of the algorithm, the user may want to carefully configure optimal register settings to detect various patterns of heart beats caused by an exercise-induced noise signal or underlying heart disease. 
 The table below lists configurable registers and a description of how these register values relate to heart beat detection. It is the purpose of this article to show the affect of these registers values on heart rate detection accuracy. 
 1. What are the configurable registers for heart beat detection? 
 RTOR_PTSF RTOR_PTSF is the fraction of the average peak value of the integrated output in the Pan-Tompkins solution where the detection threshold is set. RTOR_PTSF is a 4 bit binary number that can have values of 0-15. The value of the threshold will be the AVE_PEAK*(1+RTOR_PTSF)/16. When the integrated output of the algorithm exceeds the threshold value, the algorithm starts to search for peaks. When the integrated output drops below the threshold, the algorithm stops looking for peaks and updates its findings. RTOR_WNDW RTOR_WNDW is the number of sample averages in the integration filter, which is equivalent to the width of averaging window. RTOR_PAVG RTOR_PAVG is the number of detected peaks of the integrated output of the modified Pan-Tompkins solution used in the calculation of the average peak value. 
 Peak_Average(n)= [Peak(n) + (RTOR_PAVG-1) *Peak_Average(n-1)] / RTOR_PAVG RTOR_HOFF RTOR_HOFF (R-to-R minimum holdoff interval) is the minimum number of samples that must occur from the previous qualified R-peak for a new R-peak to be qualified. RTOR_RAVG 
 RTOR_RAVG is the R-to-R Interval averaging weight factor used in forming the average R-to-R value. 
 Interval _Average(n)= [Interval (n) + (RTOR_RAVG-1)*Interval _Average(n-1)] /RTOR_RAVG RTOR_RHSF RTOR RHSF (R-to-R interval hold off scaling factor) is the fraction of the R-to-R interval average used to qualify a new detected R-peak. If the new R-peak occurs before RR_AVE*RTOR_RHSF/8 samples then the new detected R-peak is rejected. RR_AVE is the average of R-to-R duration. RTOR_GAIN RTOR_GAIN is the post input BPF gain applied to the signal for further processing through the detection algorithm. 
 Table 1. Description of MAX30001 registers for ECG detection optimization. 
 
 2. How register value affects the outcome? 
 
 2.1 RTOR_PTSF register 
 RTOR_PTSF is the fraction factor for the average peak value of the processed output in the modified Pan-Tompkins solution and affects how detection threshold is set. RTOR_PTSF is a 4 bit number that can have values of 0-15. The value of the threshold will be determined by the formula AVG_PEAK*(1+RTOR_PTSF)/16. When the integrated output of the Pan-Tompkins solution exceeds the threshold value, the algorithm starts to search for peaks. When the integrated output drops below the threshold, the algorithm stops looking for peaks and updates its findings. 
 RTOR_PTSF and RTOR_WNDW need to be well-balanced between each other since an increased value of RTOR_WNDW will lower the amplitude of output of the differentiator-integrator. If RTOR_PTSF is set too high, the algorithm will be less likely to detect the integrator output caused by noise. If RTOR_PTSF is set too low, it will detect the noise peaks as a beat. 
 Decreased slope from SVC or PVC (example of slow sloped ECG pattern) would lower the integrator output. Therefore, it is necessary to find an optimal value for RTOR_PTSF which detects as many valid beats as possible while ignoring the noise-induced peaks. 
 
 Figure 1. PTSF =3 vs PTST=12 
 
 In example shown in Figure 1, normal heart rhythm called QRS complex is disrupted by SVC (supra ventricular contraction) four times. The amplitude of processed SVC output from the algorithm is smaller than the output of normal QRS ECG (bottom graphs). To detect both QRS and SVC correctly, it is necessary to have small PTSF value (3 in this case); PTSF value of 12 set the threshold too high and missed 4 SVC beats on right side panels. Too small PTST will make threshold close to 0 and susceptible to choose the noise spike as a beat. 
 
 2.2 RTOR_WNDW register 
 RTOR_WNDW is the number of samples used in calculation of averages in the integration filter of the modified Pan-Tompkins solution. RTOR_WNDW is a 3-bit number representing even values between 12 and 26. This allows the integration window size to be adjusted. Choosing a larger number for RTOR_WNDW increases the number of output from the integrator and results in lower/smoother amplitude; while reducing RTOR_WNDW decreases the number of output from the integrator with larger amplitude. 
 
 Figure 2. WNDW=3 vs WNDW=15 
 
 In Figure 2 example, setting WINDW =3 detected ventricular fibrillation event as separated beats as shown left bottom graph. When WINDW is changed to 15, increased integration window size makes the output from integration filter are fused as one cluster and ventricular fibrillation beats miss the detection. 
 
 2.3 RTOR_PAVG register 
 RTOR_PAVG is the number of detected peaks of the integrated output used in the creation of the average peak value. PTOR_PAVG is a 2 bit register and can have any value from {2, 4, 8, 16}. Peak average is a 1st order IIR average function. Smaller PAVG make average peak value adaptive to change quickly. 
 
 Figure 3. PAVG=0 vs PAVG=3. 
 
 In example described in Figure 3, averaged peaks of left side graphs are from 2 point peak average; while right side plot are from 16 point average. As expected, short average window made averaged value adapt quicker compared to the long average window. Linear fraction of this averaged peak value is used for the threshold for beat detection. 
 
 2.4 RTOR_HOFF register 
 RTOR_HOFF contains value for R-to-R minimum holdoff interval and it is the minimum number of samples that must occur from the past qualified R-peak for a new R-peak to be qualified. This is a 6 bit register in which the number of samples {0.. 63} required between qualified R-peaks. If sampling frequency is 125Hz and RTOR_HOFF is set to 63, this rejects any beat of new R-peaks that occur within 0ms to 504ms from a previous R-peak (63* 1000 ms /125) =504 ms ). RTOR_HOFF and RTOR_RHSF are similar methods of improving noise immunity of the algorithm. 
 
 Figure 4. HOFF=4 vs HOFF=32 
 In this example, let’s pay attention to the noise between 2 nd and 3 rd beats. When HOFF is 4, two noise peaks are classified as beats incorrectly. However only one noise peaks is classified as beat when HOFF is 32. On left graph, HOFF is too small enough to assume that new beat is detected as qualified beat even if it is not physiologically possible. Decision of HOFF value will be considered in consideration of physiological term. If device is specified to detect maximum heart rate of 250 beats per minutes, data point equivalent to 60000 ms/ 250 =240 ms should be set as HOFF. 
 
 2.5 RTOR_RAVG register 
 RTOR_PAVG is the number of detected peaks of the integrated output used in the creation of the average peak value. Same concept can be applied to create the average R-R interval value. This variable is used for internal dynamic Holdoff calculation. RTOR_RAVG is the R-to-R Interval averaging weight factor that are used in forming the average R-to-R value. RTOR_RAVG is a 2 bit registers and represents values of 2, 4, 8, 16. The R-to-R average is a 1st order IIR average function: 
 Interval _Average(n)= [Interval (n) + (RTOR_RAVG-1) * Interval _Average(n-1) ] /RTOR_RAVG 
 
 Figure 5. RAVG=2 vs RAVG=4 
 Above example shows how averaged interval is calculated with different weight factors on the past averaged interval. When R-R is varying a lot, smaller RAVG provide better tracking in the averaged interval. When RAVG is maximum, effect of past averaged interval to the current averaged interval is negligible.

2.6 RTOR_RHSF register 
 RTOR_RHSF (R-to-R interval hold off scaling factor) is the fraction of the R-to-R interval average used to qualify a new detected R-peak. RTOR_RHSF is a three bit numbers that can have values of 0 to 7. The average R-to-R duration is computed using RTOR_RRAVG samples and if the new R-peak occurs before RR_AVG*RTOR_RHSF/8 samples then the new detected R-peak is rejected. RR_AVG is average of R-to-R duration. 
 
 Figure 6. RHSF=1 vs RHSF=7 
 Let’s focus on first SVC beat (big amplitude one!). Corresponding average R-R interval is approximately 95 and internal Holdoff value would be 95*1/8 = ~12 samples. Therefore, two peaks separated by greater than 12 samples will be qualified to as valid beats and be reported. 
 If we applied the same thought on the right side, averaged R-R interval matched to first SVC beat is approximately 95 and internal Holdoff value would be 95*7/8 = ~83 samples. Raw R-R just before SVC is 60 samples (from left-top pane) which is less than Holdoff value. This is the reason why all SVCs are not detected when RHSF is equal to 7. 
 
 2.7 RTOR_GAIN register 
 
 RTOR_GAIN is the post input BPF gain applied to the signal for further processing through the detection algorithm. RTOR_GAIN is a four bit value resulting in post input BPF gain of 2^RTOR_GAIN. The input BPF is operated at full 18-bit dynamic range while downstream algorithm is implemented in 12-bit dynamic range. The mapping of from the 18-bit to 12-bit dynamic range is done through this gain setting. 
 Figure 7. GAIN=2 vs GAIN=12 (actual gain factor is 2 2 vs 2 14 ) 
 In this example of Figure.7, raw ECG is collected with ADC setting of 12 bit over 10 mV. For this simulation of MAX30001 ECG AFE, ADC setting of 17 bit over 50 mV is used. Therefore, scaling factor is as follows: (10 mV/ 2^12)/50. By multiplying ADC codes to the scaling factor, we converted ADC codes to mV unit and this is y-axis unit on the two bottom graphs. After these raw ECG values are multiplied by the RTOR_GAIN and processed by modified Pan-Tompkins, top graphs are made. Using proper RTOR_GAIN allow the user take advantage of maximizing the dynamic range of algorithm. Poor granularity of data due to low RTOR_GAIN will be likely to produce becomes incorrect detection time in high sampling rate.

Question 10

In the MAX30001 BIOZ channel, what is the phase relationship between the Demodulator Signal and the Drive Current signal?

Accepted Answer

The demodulator signal leads the drive current signal in phase. The phase by which the demodulator leads the drive current signal is set by the bits BIOZ_PHOFF[3:0] in CNFG_BIOZ (0x18).

Question 11

What is pulse transit time (PTT)?

Accepted Answer

Pulse Transit Time (PTT) is the time that it takes a pulse pressure waveform to propagate through the length of the arterial tree. This pulse pressure waveform occurs when blood is ejected from the left ventricle, and it moves with a greater velocity than the forward movement of the blood itself. PTT is used to estimate blood pressure.

PTT can be derived from calculations on electrocardiogram (ECG) and photoplethysmography (PPG) signals. Measuring PTT involves calculating the time between the R-peak of the ECG and a reference point on the pressure pulse wave measured using PPG.

Question 12

Is it possible to shift electrodes to the human body backside to measure ECG signals ?

Accepted Answer

The electrodes can be placed on the back (ensuring that you are measuring the same vector). However placing the electrodes on the back will decrease the signal amplitude and may result in unwanted movement artifact being added from the back muscles ( Infraspinatus fascia, Latissimus dorsi, etc.).

Question 13

If there is a non-negligible parasitic capacitance to GND(~100pF) on dry electrodes before the EMI filters, are there any pressing concerns with this level of capacitance on the electrodes for both ECG and BIOZ?

Accepted Answer

The 100pF shunting capacitors will not cause a significant problem. They will tend to reduce the signal received back from the body a bit, but not substantially. 
 It certainly shouldn’t be a problem for the 1nF to 10nF electrode tissue interface capacitance. This capacitance will dominate the electrode tissue interface impedance at 100KHz.

Question 14

Does the phase offset (BIOZ_PHOFF setting in MAX30001) impact overall performance (noise, in particular) in any way?

Accepted Answer

Phase offset choice does not impact overall performance.

Question 15

Is there any reliable method, without using a different FCLK, to increase the BIOZ sampling rate to about 100Hz?

Accepted Answer

Our max sampling rate for BioZ is 64SPS. We have had some customers successfully use this BioZ circuit for an impedance cardiography application; so apparently, 32Hz of Nyquist was adequate.

One can think of it like this: impedance cardiography is a use-case where one is trying to evaluate the mechanical properties of the heart; the signal range of interest (after mixing to baseband) is very similar to that of the ECG signal itself.

A 1993 paper suggested that one needed a 50Hz Nyquist; but if one looked more closely they were comparing a system with 15Hz BW to that one with 50Hz; and the 15Hz bandwidth usage was reported to have caused some attenuation and distortion. The 1993 paper is entitled “Signal fidelity requirements for deriving impedance cardiographic measures of cardiac function over a broad heart rate range” by Hurewitzh, Shyu, Lu, Reddy Schneiderman and Nagel.

A 2016 paper suggesting that using a HPF of 0.1Hz and a LPF of 20Hz gives excellent cardiographic impedance monitoring results. The 2016 paper is entitled “Design and Implementation of a Portable Impedance Cardiography System for Noninvasive Stroke Volume Monitoring” by Yazdanian, Mahnam, Edrisi and Esfahani.

Question 16

What are the effects of each of the BMUX_CG_MODE settings, and should we be using the resistive CM setting (0b11) when biasing using the internal resistors?

Accepted Answer

The reference to chopping is really a reference to the use of dynamic element switching; the use of which improves matching between the sink and source current sources. The technique used for this dynamic switching also has the nice characteristic of reducing the magnitude of 1/f noise. The reference to low pass filtering is the optional selection of an LPF within a common mode feedback circuit within the current generator that improves (i.e., increases) common mode input impedance when using a two-electrode solution (i.e., ECG and BioZ amplifiers and drive circuits use the same electrodes). The final option eliminates the common mode feedback, and instead common-mode is established using two resistors.

So, one can look at the four options as follows:

Mode 00: No chopping, thus higher 1/f noise. CFB with low-pass filter, thus high common-mode impedance at 50Hz and 60Hz

Mode 01: Chopping, thus almost no 1/f noise. CFB without low-pass filter, thus lower common-mode impedance at 50Hz and 60Hz

Mode 10: Chopping, thus almost no 1/f noise. CFB with low-pass filter, thus high common-mode impedance at 50Hz and 60Hz. Intuitively, this seems to be the best mode. However, due to the low bandwidth of the CMFB amplifier, stability of the CMFB loop is not guaranteed under all circumstances.

Mode 11: Chopping, thus almost no 1/f noise. No common-mode feedback; the common-mode is realized using two resistors. This mode has high common-mode impedance at 50Hz and 60Hz, but has lower input impedance than the CMFB amplifier, and may exceed the common-mode range for drive currents higher than 32uA due to mismatch.

We believe that the user can optimize for best matching and noise results for their particular application by selecting the option that works best for them. Consider it a toolbox for optimizing the BioZ results.

Question 17

What is HRV (Heart Rate Variability)?

Accepted Answer

HRV also known as Heart Rate Variability is simply a measure of the variation in time between each heartbeat. This variation is controlled by a primitive part of the nervous system called the autonomic nervous system (ANS).

A high HRV is an indicator of greater cardiovascular fitness and be more resilient to stress. It can be measured by some additional signal signal processing on ECG data.

Question 18

What would be a good algorithm to measure heart rate in a noisy environment with a lot of artifacts?

Accepted Answer

An HR algorithm based on short-term autocorrelation center clipping method is a very robust algorithm that is capable of extracting data from noisy environments.

You may find the following link to be very educational: http://przyrbwn.icm.edu.pl/APP/PDF/118/a118z1p31.pdf

Question 19

Why we need to connect a 1uF capacitor between CAPP and CAPN in MAX30001 design?

Accepted Answer

The 1uF capacitor between CAPP and CAPN pins is used to create a 0.5Hz high-pass fliter response in ECG channel.

Question 20

Should I use MAX30001 or MAX86150 as ECG AFE for DRY ELECTRODES?

Accepted Answer

It depends on the applications and use case i.e the location to which you are measuring and not just dry electrodes. The MAX86150 was designed for finger-based applications, and only supports two electrode configurations. There are no claims of medical grade accuracy or IEC/FDA compliant performance. It was made for BioID. On the other hand if your application is a wrist-based solution, with one electrode touching the wrist, the MAX30001 is better due to the added benefit/option of a 3 rd electrode. Additionally, the MAX30001 is marketed as having Clinical/Medical grade accuracy, enabling medical type applications but also in difficult locations like the wrist + finger.

The only time MAX86150 is better than MAX30001 is if you want PPG and ECG in synchronization, and are using Finger-to-Finger ECG measurements. But there is still the medical grade accuracy questions. Overall, considering all possible applications, the MAX86150 can be considered as being appropriate for only a subset of applications compared to the superset that the MAX30001 can handle. That means, when in doubt, use the MAX30001.

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MAX30001CTI+T: FAQ