A DAQ of All Trades: Another Configuration for Bipolar Inputs

Blog Post created by tschmitt Employee on Aug 29, 2017

Welcome back to the ADAQ798x ADC driver configuration blog series! In today’s post, we’re going to look at the difference amplifier configuration, another means of interfacing the ADAQ798x with bipolar input signals. This configuration can be used for bipolar signals with wide input voltage ranges and bandwidths. We’ll see how to select the required external components for any given input range and how they affect other specifications like input impedance, noise, and dc errors.


The Difference Amplifier

The ADC driver can be configured as a difference amplifier using four external resistors, shown below:


This configuration can be thought of as a superposition of the non-inverting and inverting configurations; the bipolar input signal is multiplied by the amplifier’s inverting gain, while the dc bias voltage (using VREF, for reasons discussed in previous posts) is multiplied by the non-inverting gain. CN-0393 utilizes this configuration to condition a ±10 V output swing from a PGIA (AD8251). The transfer function for this configuration is:


The first step is to find the appropriate ratio of Rf and Rg, which is determined by the ratio of the input amplitude (ΔvIN) to the full-scale range of the ADC (0 V to VREF):


Unlike with the non-inverting configurations we’ve discussed, the signal gain can be less than 1, so we don’t need to make any modifications (i.e. additional resistors) to attenuate input signals with amplitude larger than VREF. It’s worth noting that the signal does get inverted from the input to the output.


R1 and R2 are then used to attenuate VREF such that the output of the ADC driver is biased to the ADC midscale (VREF/2). The ratio of R1 and R2 is determined by the ratio of Rf and Rg:


The above also assumes that the design is utilizing VREF as the dc input voltage tied to R1.


After finding these ratios, we then need to select specific values for each of the resistors. There are a few considerations to make before we start blindly selecting components:


First, the value of Rf can affect the ADC driver’s stability. If Rf becomes too large, the noise gain frequency response will start peaking, and can become unstable (as described in MT-050). As we mentioned several posts ago in "Adding Gain for Unipolar Inputs", Rf should be limited to prevent this from occurring.


Also, as we saw in our previous post, "Attenuating Bipolar Inputs", larger resistors will result in more system noise. This configuration is more susceptible to noise issues than the one we discussed last week, because the ADC driver’s noise gain will always be larger than 1. The Noise Considerations and Signal Settling section in the ADAQ7980/ADAQ7988 data sheet and the System Noise Analysis section in CN-0393 shows how to quantify the system noise for this configuration.


And still another consideration is the resistors effect on the system offset error. The resistors will interact with the ADC driver’s input bias current to create an offset error at its output. This effect becomes more pronounced as their resistances increase. According to MT-038, in order to mitigate this effect, the parallel combination of R1 and R2 must be equal to that of Rf and Rg.


Let’s consider an example where vIN is ±1.25 V and VREF = 5 V. Using the equations above, we find that Rf must be 2×Rg, and R1 must be 5×R2. If we want to ensure that the input bias currents don’t create system offset error, the parallel combinations of R1||R2 and Rf||Rg must be equal as well, which R1 = 0.8×Rf. If we select Rf = 2 kΩ, for example, we need Rg = 1 kΩ, R1 = 1.6 kΩ and R2 = 320 Ω.


Closing Thoughts

The difference amplifier configuration is capable of interfacing the ADAQ798x with bipolar signals with many amplitude and frequency ranges, and it is remarkably simple to design. There are a couple other things to watch out for, though.

First, remember that some applications are concerned with achieving a high input impedance. With the other configurations we’ve discussed, this is possible by increasing the resistor values (and reducing the input bandwidth to take care of the additional noise). This configuration struggles to do this, however, as Rf and Rg can’t be too large that they impact ADC driver stability. The input impedance of this circuit is equal to that of an inverting amplifier:


To achieve an input impedance of 1 MΩ for example, Rg would need to be 1 MΩ, and Rf will likely be too large for the ADC driver to function correctly (at least when using common gains). The only practical way to increase the input impedance of the system would be to use another signal conditioning stage in front of the ADC driver.


The bright side is that since this configuration will likely feature smaller resistors, it is less likely that it’ll require extra filtering to compensate for resistor noise. Also, this makes balancing the offsets created by the input bias current more practical, as R1 and R2 can be easily selected to balance the offset from Rf and Rg. These two qualities allow this configuration to achieve higher levels of precision and higher signal bandwidths than the non-inverting configurations that could achieve higher input impedance.


Also worth mentioning is that this configuration can be used more easily in single-supply applications where the ADAQ798x’s negative supply is tied to ground. This is because the amplifier’s inputs are held at a constant dc voltage, and there’s less concern of violating the input common mode voltage specifications (shown in the ADAQ7980/ADAQ7988 data sheet).


Thanks again for joining me in this blog series! In our next and final entry, we’re going to look at an active filtering configuration for the ADAQ798x. 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!