[Part 1 offers an introduction to selecting op amps for use with data converters. Part 2 looks at key ADC/DAC specifications. Part 3 examines the critical issues involved in selecting op amps for driving ADC inputs.]
Driving ADC/DAC Reference Inputs
It might seem odd to include a section on voltage references in a book devoted primarily to op amp applications, but the relevance will shortly become obvious. Unfortunately, there is little standardization with respect to ADC/DAC voltage references. Some ADCs and DACs have internal references, while others do not. In some cases, the dc accuracy of a converter with an internal reference can often be improved by overriding the internal reference with a more accurate and stable external one.
Although the reference element itself can be either a bandgap, buried zener, or XFET (see Reference 1), practically all references have some type of output buffer op amp. The op amp isolates the reference element from the output and also provides drive capability. However, this op amp must obey the general laws relating to op amp stability, and that is what makes the topic of references relevant to the discussion. Figure 3-47 summarizes voltage reference considerations.
Figure 3-47: ADC/DAC voltage reference considerations
- Data converter accuracy determined by the reference, whether internal or external
- Bandgap, buried zener, XFET generally have on-chip output buffer op amp
- Transient loading can cause instability and errors
- External decoupling capacitors may cause oscillation
- Output may require external buffer to source and sink current
- Reference voltage noise may limit system resolution
Note that a reference input to an ADC or DAC is similar to the analog input of an ADC, in that the internal conversion process can inject transient currents at that pin. This requires adequate decoupling to stabilize the reference voltage. Adding such decoupling might introduce instability in some reference types, depending on the output op amp design.
Of course, a reference data sheet may not show any details of the output op amp, which leaves the designer in somewhat of a dilemma concerning whether or not it will be stable and free from transient errors. Fortunately, some simple lab tests can exercise a reference circuit for transient errors, and also determine stability for capacitive loading.
Figure 3-48 shows the transients associated with the reference input of a typical successive approximation ADC. The ADC reference voltage input must be stabilized with a sufficiently large decoupling capacitor, in order to prevent conversion errors. The value of the capacitor required as CB may range from below 1 F, to as high as 100 F. This capacitor must of course have a voltage rating greater than the reference voltage. Physically, it will be of minimum size when purchased in a surface-mount style.
Figure 3-48: Successive approximation ADCs can present a transient load to the reference
Note that in this case, a 1 F capacitor on the reference input is required to reduce the transients to an acceptable level. Note that the capacitor size can be electrically larger for further noise reduction—the tradeoff here is of course cost and PCB real estate. The AD780 will work with capacitors of up to 100 µF.