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ADC Basics, Part 6: Key Op-Amp DC & AC Specifications

If you are building a single-supply, 12-bit SAR-ADC system, such as a multiplexed circuit, handheld meter, data logger, automotive systems, or some other monitoring system, you probably will require an analog signal-conditioning front end. These signal conditioning circuits usually have an amplifier and/or a programmable gain amplifier (PGA) to accomplish analog signal-gain, filtering, and driving activities. A typical block diagram is shown below.

Typical signal chain path for circuits using SAR-ADCs.

Typical signal chain path for circuits using SAR-ADCs.

We can discuss the attributes and characteristics of various single-supply operational amplifiers (op-amps) and their related specifications, but you probably have seen this style of review before. And you certainly can count on the various manufacturers to point out the exceptional features of their products. The purpose of this article is to point out the key amplifier characteristics as they relate to their position in these single-supply systems.

The amplifiers that fit into this class of application usually are single-supply, voltage-feedback ones with possible special functions such as programmable gain or shutdown. For our systems, we are going to look at the standalone op-amp and the PGA, excluding the amplifier(s) for the anti-aliasing filter and the driving amplifier to the ADC. The fundamental DC and noise characteristics of interest are:

  • Input offset voltage/over temperature
  • PGA gain range/gain error
  • Bandwidth
  • Output noise

These specifications enable a quick decision on the components in these circuits before breadboard. The dynamic range output code of a 12-bit ADC spans from 0 to 4095. We will see how a majority of the amplifier's output range characteristics affect the 12-bit ADC's dynamic range.

Input offset voltage
The amplifier and PGA's input offset voltage and offset voltage drift occur because of mismatches in the internal amplifier's input stage. Within an op-amp, there is a differential pair of transistors in the input structure between the inverting and non-inverting inputs. In this evaluation, we will use CMOS op-amps.

You can find these offset specifications in the product data sheets. In this evaluation, one multiplies these errors by the amplifier's gain. This places the op-amp and ADC errors at the input of the ADC.

PGA gain range/gain error/over temperature
The amplifier's gain error results from the inaccuracy of the input and feedback resistors. Typically, accuracy of these resistors can be 1 percent or, better yet, 0.1 percent. The resistor manufacturer also specifies the resistor's drift over temperature.

The PGA gain error and over temperature errors are the result of the mismatch of the resistor values in the PGA chip. You can find the specifications for the PGA's gain error and over temperature gain error in the product data sheet.

As a first step in this evaluation, we combine the offset and gain errors, along with the ADC differential nonlinearity (DNL) and integral nonlinearity (INL). You can tabulate these errors in an Excel spread sheet, like the one shown below with the circuit diagram for those numbers.

An Excel spreadsheet used to calculate DC errors such as offset and gain. An equivalent 12-bit error is equal to the total unadjusted error (TUE) divided by the gain. TUE is equal to the root sum square of all the errors.

An Excel spreadsheet used to calculate DC errors such as offset and gain. An equivalent 12-bit error is equal to the total unadjusted error (TUE) divided by the gain. TUE is equal to the root sum square of all the errors.

This 12-bit system uses a programmable gain amplifier to provide analog gain to the system. The op-amp follows the PGA to provide ample drive to the 12-bit ADC.

This 12-bit system uses a programmable gain amplifier to provide analog gain to the system. The op-amp follows the PGA to provide ample drive to the 12-bit ADC.

If accuracy is of no concern, these DC errors impact only the total dynamic range of the 12-bit converter near the rails. You will see this effect below.

Bandwidth
The bandwidth ranges of the analog devices are critical. These values service the bandwidth requirements of the input signals, and they provide enough frequency range to accommodate the clocking delays in the system. The next article in this series will provide an overview of the clocking delays found in the systems under evaluation. The objective of the designs in this series is to maximize the bandwidths of five proposed systems.

Output noise
Amplifier noise primarily originates in the input stage transistors. You describe the amplifier output noise using regions of frequency. However, it is more effective to describe this noise in terms of voltage (either RMS or p-p) and examine total cumulative noise at the amplifier's output. One calcultates this output noise by integrating the total noise over frequency from DC to the system's maximum bandwidth. Amplifier noise spans across the entire output dynamic range, and this error is impossible to calibrate.

Noise is considered an AC phenomenon. However, in this series, we compare the system's noise impact to the total DC specifications in the circuit. You can see this concept in the image below, which shows the DC errors and output noise across the complete 12-bit range. This figure quantifies the ADC dynamic range.

When accounting for DC errors, the gain error and offset errors of all analog and mixed-signal devices are placed near the rail. The noise errors span across the entire dynamic range.

When accounting for DC errors, the gain error and offset errors of all analog and mixed-signal devices are placed near the rail. The noise errors span across the entire dynamic range.

Note that the DC errors (offset, gain error, etc.) appear near the supply voltage or ground. In an individual system, these errors may land between the supply rails. However, in our evaluation, the diagram rightfully places these errors at either of the two rails. Both the DC errors and noise errors impact the dynamic range of the entire system.

There are various attributes and characteristics of single-supply op-amps that service applications such as handheld meters, data loggers, and monitoring systems. We have covered the main attributes, which will help you make a first pass judgment on which amplifiers will suit your needs. The specifications of interest are the offset voltage, gain error, bandwidth, and noise errors. We suggest that you tabulate these specifications in an Excel spreadsheet for further reference.

Next month's article will introduce the first systems under consideration and look at the timing effects. Here are some additional sources of information on this topic:

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