First rule of thumb when driving ADC inputs

This technical article was updated on July 23, 2020.

Engineers like to simplify the design process using many rules to get the design going in the right direction before optimizations occur. One of my favorites is to always drive the inputs of an analog-to-digital converter (ADC) with a low impedance source. Why do I like this one so much? Because it offers many benefits for a precision data acquisition block.

Let’s start by reviewing a common application in which a high-voltage signal source needs to be level translated to the required ADC input range. The simple voltage divider in Figure 1 could solve this problem of level translating a +/-5V signal to a 0-5V. The equivalent impedance R_eq of the voltage divider is the parallel combination of R1 and R2.

So how does this finite source impedance affect your data acquisition system?

Figure 1

High source impedance introduces both linear and non-linear errors during the data acquisition. Some of the major errors resulting in poor SNR and THD of the data acquisition are:

• Gain error: Source impedance at the input of the ADC forms a voltage divider with the input impedance of the ADC. This input voltage drop across the source impedance introduces a gain error in the measurement. Keeping the source impedance low will help keep this systemic error small.
• Settling time error: To make a precise conversion, the ADC inputs need to be settled within the available acquisition time of the data converter. The sampling capacitor inside the ADC is only allowed to charge during this acquisition time. The source impedance at the inputs of the ADC, along with the input capacitor of the ADC, creates additional time constants. Thus, this low-pass filter would introduce errors in the sampled signal due to settling errors.
• Distortion: The low-pass filter I mentioned above that’s created by the source impedance and the input capacitance also introduces a signal-dependent distortion during the conversion. The distortion results from the inherent non-linearity in capacitors, which depends on the voltage stored across them. The input current used to sample the input signal creates an error voltage across the source impedance. For a sine wave input signal, this error contains harmonics, and degrades the distortion of the system.

The impact of source impedance can be significant when driving the ADC inputs in a precision data acquisition system. Hopefully, this rule helps – always drive the inputs of an ADC with a low impedance source.

To get some more recommendations on how to drive ADC inputs, you might want to check out this TI Precision Design, which provides techniques for extracting the highest performance from your ADC: Data Acquisition Optimized for Lowest Distortion, Lowest Noise, 18bit, 1MSPS Reference Design.

Now that I’ve shared my favorite rule of thumb, what are some of yours when driving an ADC? I hope you’ll tell us below.