Using Op Amps with Data Converters - Part 3

Introduction
Op amps are often used as drivers for ADCs to provide the gain and level-shifting required for the input signal to match the input range of the ADC. An op amp may be required because of the antialiasing filter impedance matching requirements. In some cases, the antialiasing filter may be an active filter and include op amps as part of the filter itself. Some ADCs also generate transient currents on their inputs due to the conversion process, and these must be isolated from the signal source with an op amp. This section examines these and other issues involved in driving high performance ADCs.

To begin with, one shouldn't necessarily assume that a driver op amp is always required. Some converters have relatively benign inputs and are designed to interface directly to the signal source. There is practically no industry standardization regarding ADC input structures, and therefore each ADC must be carefully examined on its own merits before designing the input interface circuitry. In some applications, transformer drive may be preferable.

Assuming an op amp is required for one reason or another, the task of its selection is a critical one and not at all straightforward. Figure 3-21 lists a few of the constraints and variables. The most important requirement is that the op amp should not signifi cantly degrade the overall dc or ac performance of the ADC. At first glance, it would appear that a careful comparison of an op amp data sheet with the ADC data sheet would allow an appropriate choice. However, this is rarely the case.

• Minimize degradation of ADC/DAC performance specifications
• Fast settling to ADC/DAC transient
• High bandwidth
• Low noise
• Low distortion
• Low power

• Note: Op amp performance must be measured under identical conditions as encountered in ADC/DAC application
    - Gain setting resistors
    - Input source impedance, output load impedance
    - Input/output signal voltage range
    - Input signal frequency
    - Input/output common-mode level
    - Power supply voltage (single or dual supply)
    - Transient loading

Figure 3-21: General op amp requirements in ADC driver applications

The problem is that the op amp performance specifications must be known for the exact circuit confi guration used in the ADC driver circuit. Even a very complete data sheet is unlikely to provide all information required, due to the wide range of possible variables.

Although the op amp and ADC datasheets should definitely be used as a guide in the selection process, it is unlikely that the overall performance of the op amp/ADC combination can be predicted accurately without actually prototyping the circuit, especially in high performance applications.

Various tested application circuits are often recommended on either the op amp or the ADC data sheet, but these can become obsolete quickly as new op amps are released. In most cases, however, the ADC data sheet application section should be used as the primary source for tested interfaces.

Op Amp Specifications Key to ADC Applications
The two most popular applications for ADCs today are in either precision high-resolution measurements or in low distortion high speed systems. Precision measurement applications require ADCs of at least 16 bits of resolution, and sometimes up to 24 bits. Op amps used with these ADCs must be low noise and have excellent dc characteristics. In fact, high resolution measurement ADCs are often designed to interface directly with the transducer, eliminating the need for an op amp entirely.

If op amps are required, it is generally relatively straightforward to select one based on well understood dc specifications, as listed in Figure 3-22.

• Dc
    - Offset, offset drift
    - Input bias current
    - Open loop gain
    - Integral linearity
    - 1/f noise (voltage and current)

• Ac (Highly Application-Dependent)
    - Wideband noise (voltage and current)
    - Small and Large Signal Bandwidth
    - Harmonic Distortion
    - Total Harmonic Distortion (THD)
    - Total Harmonic Distortion + Noise (THD + N)
    - Spurious Free Dynamic Range (SFDR)
    - Third-Order Intermodulation Distortion
    - Third-Order Intercept Point

Figure 3-22: Key op amp specifications

It is much more difficult to provide a complete set of op amp ac specifications because they are highly dependent upon the application circuit. For example, Figure 3-23 shows some key specifications taken from the table of specifications on the data sheet for the AD8057/AD8058 high speed, low distortion op amp (see Reference 1). Note that the specifications depend on the supply voltage, the signal level, output loading, and so forth. It should also be emphasized that it is customary to provide only typical ac specifications (as opposed to maximum and minimum values) for most op amps. In addition, there are restrictions on the input and output CM signal ranges, which are especially important when operating on low voltage dual (or single) supplies.

Most op amp datasheets contain a section that provides supplemental performance data for various other conditions not explicitly specified in the primary specification tables. For instance, Figure 3-24 shows the

AD8057/AD8058 distortion as a function of frequency for G = +1 and V_S = 5 V. Unless it is otherwise specified, the data represented by these curves should be considered typical (it is usually marked as such).

Note however that the data in both Figure 3-24 (and Figure 3-25) is given for a dc load of 150 Ω. This is a load presented to the op amp in the popular application of driving a source and load-terminated 75 Ω cable. Distortion performance is generally better with lighter dc loads, such as 500 Ω - 1000 Ω (more typical of many ADC inputs), and this data may or may not be found on the data sheet.

On the other hand, Figure 3-25 shows distortion as a function of output signal level for a frequencies of 5 MHz and 20 MHz.

Whether or not specifications such as those just described are complete enough to select an op amp for an ADC driver application depends upon the ability to match op amp specifications to the actually required ADC operating conditions. In many cases, these comparisons will at least narrow the op amp selection process. The following sections will examine a number of specific driver circuit examples using various types of ADCs, ranging from high resolution measurement to high speed, low distortion applications.