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A DAQ of All Trades: Adding Gain for Unipolar Inputs

Blog Post created by tschmitt Employee on Aug 8, 2017

Welcome back to the ADAQ798x ADC driver configuration blog series! As we discussed last time, we will be looking at several common and useful configuration options for the ADAQ798x’s integrated ADC driver, how to design them, and what to watch out for when doing so. In today's entry, we will discuss how to use the common non-inverting configuration to interface the ADAQ798x with unipolar input sources that are smaller than the ADC’s input range of 0 V to VREF.

 

The Non-inverting Configuration

 

Recall from our last post that the ADC converts inputs between 0 V and VREF. This means the ADC driver’s output must also be 0 V to VREF to allow the system to utilize the full range of 216 codes that the ADAQ798x provides. The ADAQ798x’s integrated ADC driver can provide gain to give the necessary boost to signals with smaller amplitudes.

 

That’s where the non-inverting configuration comes in! This configuration provides gain for unipolar signals, a high input impedance, and requires only two additional resistors.

 

 

Many system designers already know how the non-inverting configuration works, but we’ll look at it here in the context of the ADAQ798x, and how the configuration can impact key system performance parameters, including system noise, signal-to-noise ratio (SNR) and total harmonic distortion (THD).

 

First, how do we select the resistors Rf and Rg given the application’s input range and reference voltage? The voltage at the output of the ADC driver (vAMP_OUT) is:

 

 

Given that vAMP_OUT is between 0 V and VREF, the ratio of Rf to Rg is easy to calculate for the application’s input range (vIN+):

 

 

After calculating the Rf to Rg ratio, their specific values must be selected. The “right” value of these resistors depends on the application, and must be chosen to balance system noise performance with power dissipation, distortion, and amplifier stability. Lower values of Rf result in lower noise, but also results in more current drawn from the ADC driver’s output (increasing power consumption). Using higher values of Rf can limit this power consumption, but results in added system noise and potential stability issues.

 

The amount of noise a resistor generates is proportional to its resistance. Large resistors contribute more noise, and can impact the system’s noise floor and ac performance specs (like SNR). The total system noise can be calculated by taking the root sum square of the individual noise sources in the circuit, including the resistors, the ADC driver, and the ADC itself:

 

 

where vn,system is the system rms noise floor, vn,ADC driver is total noise of the ADC driver circuit (including the external resistors), and vn,ADC is the ADC’s noise floor specification.

 

The ADAQ7980/ADAQ7988 data sheet explains how to calculate the vn,ADC driver (see Noise Considerations and Signal Settling), and gives vn,ADC as 44.4 μVRMS for a 5 V reference. CN-0393 also explains how to calculate the expected SNR of the system based on the total system noise (see System Noise Analysis). In the interest of brevity, we won’t do those calculations again here, but we’ll give another example here.

 

Let’s look at a case where the ADAQ7980 needs to interface directly with a sensor with an output range from 0 V to 2.5 V while using a 5 V reference. Since the sensor’s output amplitude is equal to half the ADC’s input range, the ADC driver should be set in a gain of 2. This requires Rf to be equal to Rg, but the selection of Rf is somewhat flexible. First, let’s look at how different values of Rf (and Rg) affect the system’s noise floor and corresponding expected SNR:

 

Rf, Rg (Ω)

vn,system (μVRMS)

Calculated SNR (dBFS)

500

53.3

90.4

1k

54.3

90.2

2k

56.4

89.9

5k

62.2

89.0

 

As you can see, the system noise will increase and SNR will degrade when using higher values of Rf and Rg. Increasing gain will also degrade SNR performance because it increases the effective noise contributed by the ADC driver’s input voltage noise and Rg. The plots below show measured results for SNR and THD (total harmonic distortion) for various gains using Rf = 1 kΩ (input frequency = 10 kHz).

 

 

 

One of the down sides of selecting smaller resistors, however, is that the ADC driver needs to deliver more current (and therefore power) through the feedback network. The instantaneous current going through Rf and Rg is vAMP_OUT divided by the sum of Rf and Rg. This current adds to the total power dissipation of the system, and should be limited in low-power applications.

 

Closing Thoughts

One benefit of this configuration is that its input impedance is very large, since the source is tied directly to the non-inverting node of the ADC driver. This is especially useful for sources with significantly large output impedance. We’ll see that this is not always the case for other configurations.

 

Although the non-inverting configuration can provide gain, there are some practical limitations. First, as stated in this Analog Dialogue article, the ADC driver must maintain a certain large- and small-signal bandwidth to achieve forward- and reverse-settling requirements of the ADC. Bandwidth is inversely proportional to closed-loop gain. The system noise also increases with gain, and at a certain point will degrade performance too much to be viable without considerable filtering (which we’ll cover later in this series).

 

Also, for applications that require very low offset and gain error and drift, be sure to use precision resistors with adequate tolerance and TCR specifications. If possible, use matched resistor networks that specify the tracking of resistance and TCR between their individual resistors (for example, the LT5400 series). CN-0393 explores this concept in greater detail. Note also that the ADC driver’s input bias current will flow through Rf and Rg, which will create voltage offsets in the system. A resistor can be placed between the non-inverting node of the ADC driver and the input source to balance out these offsets, but remember that this resistor will also add noise to the system! (See MT-038 for more information.)

 

Thanks for reading! Next week we’ll look at two of the ADC driver configurations designed to allow the ADAQ798x to interface with bipolar input signals. Follow the EngineerZone Spotlight to be notified when the next addition to this series is available!

 

Have any questions? Feel free to ask in the comments section below!

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