Welcome back to the ADAQ798x ADC driver configuration blog series! In today’s entry, we’ll look at one of the configurations that can be used to interface the ADAQ798x to bipolar sensors and input sources. These types of signals are common in industrial and data acquisition applications. This configuration builds on the non-inverting configuration we discussed previously to convert a bipolar signal to a unipolar one for the integrated ADC.

**The Non-inverting Summing Configuration**

Bipolar signals swing above and below ground (0 V). Since the ADAQ798x’s integrated ADC can only convert signals between 0 V and V_{REF}, bipolar signals need to be dc-biased and properly scaled for the ADC. The following configuration accomplishes this by adding two resistors (R_{1} and R_{2}) to the standard non-inverting configuration.

This configuration performs bipolar to unipolar conversion by summing the input signal with a separate dc voltage to bias the ADC driver’s output to the ADC’s midscale input (V_{REF}/2). Using the reference source (V_{REF}) as the dc voltage is often practical, as it eliminates the need for additional circuitry (the ADAQ798x is always accompanied by a reference source anyway!). It also prevents deviations in V_{REF} from adding offset error to the system, since the ADC driver’s dc bias will always be half of V_{REF}. For these reasons, we will look specifically at this configuration utilizing V_{REF} as the dc “shifting” voltage.

The transfer function for this configuration is:

Similarly to the regular non-inverting configuration, the ratio R_{f} and R_{g} determines the gain from IN+ to AMP_OUT, but this ratio now depends on the input amplitude of v_{IN} as well. Note that v_{IN }is bipolar, but the voltage on the non-inverting node is unipolar. That means that for the minimum value of v_{IN}, the voltage on IN+ must be 0 V:

This relationship gives the ratio of R_{1} to R_{2}:

R_{f} and R_{g} can be determined using the configuration’s transfer function and the condition that the output of the ADC driver (v_{AMP_OUT}) is equal to V_{REF}/2 when v_{IN} is 0 V. Solving this equation for R_{f} and R_{g} gives:

We now have the ratios of R_{1} to R_{2} and R_{f} to R_{g}, but we still need to pick specific values. We addressed selecting R_{f} and R_{g} values in our previous post. R_{1} and R_{2} selection should be determined based on the application’s noise, accuracy, and input impedance requirements. Small resistances will improve noise and can reduce offset errors caused by its interaction with the ADC driver’s input bias current (see MT-038 and CN-0393), but large resistances are required to increase input impedance and reduce the output current of the reference source. The input impedance of this circuit is:

Note that for the specific case where the amplitude of v_{IN} is ±V_{REF}, the ratio of R_{f} to R_{g} is 0. In this case, the ADC driver gain is 1, meaning R_{g} is omitted and R_{f} can be 0 Ω.

Let’s look at an example where the ADAQ7980 needs to perform bipolar-to-unipolar conversion of a ±1 V input signal, with V_{REF} = 5 V and using R_{f} = 2 kΩ. Using the above equations, R_{2} must be 5 times R_{1} and R_{f} must be 2 times R_{g}. Since R_{f} is 2 kΩ, R_{g} must be 1 kΩ. Specific values of R_{1} and R_{2} can be selected depending on the application’s requirements. For this example, we’ll aim to select a combination of R_{1} and R_{2} that negates the effects of the input bias current on offset error. MT-038 explains that R_{1}||R_{2} should be equal to R_{f}||R_{g} to achieve this, which gives R_{1} = 800 Ω and R_{2} = 4 kΩ.

But let’s also consider an example where v_{IN} = ±10 V with V_{REF} = 5 V. In this case, we run into a problem where the ratio of R_{f} and R_{g} is a negative number, so we can’t actually achieve this input range with this configuration. In fact, the largest v_{IN} that will work with this configuration is ±V_{REF}, where the ADC driver gain is equal to 1. Luckily, we’ll be looking at two other configurations that will allow us to extend past this input range in future entries to this series!

**Closing Thoughts**

The above configuration can also be used for unipolar signals by connecting R_{2} to ground instead of V_{REF}. This modification is useful for unipolar input signals that need to be attenuated for the ADC (with amplitude >V_{REF}). In this case, the ADC driver will most likely be in unity gain, so R_{f} and R_{g} are not necessary.

As mentioned above, if the application requires a high input impedance, R_{1} and R_{2} must be large, which can increase the noise floor of the system. We can compensate for the increased noise with the addition of a shunt capacitor and/or by oversampling and decimating. Both of these options sacrifice input signal bandwidth to reduce the noise floor. For low-bandwidth or dc applications, however, the input bandwidth is not as important. For this reason, these configurations are better suited for low-bandwidth, high input impedance applications. We'll discuss this in more detail in our next post.

One problem this does not address, however, is the offset error caused by the ADC driver’s input bias current flowing across the resistors. Large resistances result in large dc errors. This error can be reduced at the expense of some input range by adjusting the ratio of R_{1} and R_{2} to compensate for the undesired voltage drop, or by selecting R_{f} and R_{g} values that cancel out the offset caused by R_{1} and R_{2}. However, keep in mind that R_{f} must be small enough to ensure amplifier stability, so the second option is not always viable.

Thanks again for joining me in this blog series! Next week, we’ll look at another modification to the non-inverting configuration designed for use with bipolar input signals that are too large for the one we discussed today. 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!