Purpose: The ACLO block within the ADAS1000 is used to help user identify if an electrode has fallen off or if the connection is poor (higher impedance). This document captures some detail around the operation of the block and using it.
Implementation: Inject an AC current into each electrode and look for the signal in the digital data
- A properly connected electrode will have a very small signal as the drive current flows into the Right Leg (RL).
- A disconnected electrode will have a larger signal as determined by a capacitive voltage divider (source and cable capacitance)
The circuit includes a DAC to drive the inphase/anti-phase sinusoidal signals at a fixed frequency of 2.039kHz. Each signal has a nominal peak-to-peak amplitude of 2V centered at +1.3V relative to the ADAS1000's AGND/DGND.
The ACLO circuit drives the 2.039kHz sinusoidal current into each electrode, it's a voltage DDS ac-coupled to each ECG pin.
The ECG channel is digitized through the ADC channel, this includes the ACLO content. Each channel is I/Q demodulated and amplitude detected. There's sqrt(sum-of-squares) processing to find the amplitude without having to worry about the phase shift of the received signal relative to the drive. The resulting amplitude is low-pass filtered and sent to the digital threshold detectors. The AC lead off detection occurs solely in the digital domain. If the resulting amplitude exceeds the programmed thresholds, a flag is asserted. The user also has the ability to read back the amplitude result from each channel.
The thresholds are programmable (LOFFUTH – upper threshold) and (LOFFLTH – lower threshold). This pair of thresholds applies to all channels within the device.
The use of the upper threshold level should be clear - if the signal is bigger than some value, the impedance is "high" so a wire is probably off or the electrode contact is degrading. Some experimentation will be required to identify the appropriate threshold based on the particular cable/electrode/protection scheme as these parameters will typically be unique for the use case. This might take the form of starting with a high threshold and ratcheting it down until they tripped, then increase it by some safety margin. This would give simple dynamic thresholding that automatically compensates for many of the circuit variables.
The lower threshold was added for the case where the customer uses ACLO only, but for the event where an electrode cable has been off for a long time and the DC voltage has saturated to a rail, or maybe the electrode cable has somehow shorted to a supply. In either case there may be no AC signal at all, yet the electrode may not be connected. The lower threshold checks for some minimum signal level.
In addition to the AC Lead off flag, there is also a voltage measurement available on a per channel basis (LOAMxx registers).
ADC Out of Range:
There may be instances when using the ACLO function and using a common mode reference with one or more electrodes, where the lead off flag may not function properly. This situation may result in saturation of the input amplifiers and the ADC, resulting in the ADC outputting “out of range data” with no carrier to the leads off algorithm. The algorithm would then report little or no ac amplitude. The ADAS1000 contains flags to indicate if the ADC data is out of range – indicating a hard electrode off state. The ADC out of range flag is contained in the Header word (bit 20 in 0x40).
Things to note:
- AC-lead-off results are based on ECG channel data which may be a differential measurement (Vector mode).
- Channel Gain (x1.4, x2….) and channel configuration (analog lead or not) have a direct effect on the ECG samples and therefore have a direct effect on the ac-lead-off results. Higher channel gains will result in higher codes in the ac-lead-off results.
- When the contact impedance is low (ideal contact), the ac-lead-off current (whose carrier is at 2KHz) will result in a low voltage. When the contact impedance is high (poor quality), the ac-lead-off current will result in a high detectable voltage.
- When an electrode is completely disconnected, the ecg input pin will float, depending on leakage currents, it may float towards the supply rail. In the event that it does float to the supply rail, the ECG channel will be saturated and the ACLO amplitude result would then return 0 or close to 0 - making it look like the electrode is actually connected. Therefore, in addition to monitoring the AC LO Flag, user should also monitor the ADC-OUT-OF-RANGE flags. The electrode should be considered as off when either the ADC is flagged as out of range or the ac-lead-off flag is set.
- ESIS protection network and defibrillator protection networks load the circuit and have a direct effect on the sensitivity of the ac-lead-off circuit.
- ACLO detection operates at 128kSPS data rate.
ACLO and Common Mode Configuration
- One electrode with poor contact will have an effect on the ac-lead-off results of all the ECG channels it contributes to (directly or via RLD or via COM_IO)
- In the example shown, the LL electrode is OFF, this has the following affect on the other leads
- In this scenario, user should identify the electrode and remove from the RLD mix and COM_IO mix. This requires careful detection threshold setting and may even require a smart algorithm in the host to play a little bit with the RLD and COM_IO blocks before determining which electrode degraded or fell off.
Results from ADI Evaluation Board
Configuration: Measurements on RA electrode using ADI evaluation board. All other Channels were disabled. Measurements were made at the device pin - with no external filtering/cable included.
A programmable resistor was placed between the RA electrode and VCOM (1.3V). The ACLO amplitude measurement was recorded, the corresponding impedance calculated and plotted versus applied resistance. This plot is for Gain = 1.4.