The differential outputs provided by fully differential amplifiers (FDA) can be used in a positive feedback scheme to generate a partially active output impedance. Applying this technique, to set the isolating resistor in a typical RC filter at a Successive Approximation Register (SAR) ADC input, provides an ancillary advantage. Many designs buffer the FDA input resistors with precision low power Rail-to-Rail (RR) op amps. These buffers are often much lower in bandwidth than the requisite FDA bandwidth to drive the SAR sampling events. The system harmonic distortion limits might well rest at the outputs of these buffers if asked to drive too low an input resistor as part of the FDA design. One outcome of implementing an active output resistance in the FDA is to also scale up the required input resistor to hit the same gain. This technique can be used to hit the same design targets providing lighter loading on the lower speed input stage buffers possibly improving the overall system THD. The requisite design equations for the FDA and example designs will be shown.
Low power, high impedance buffered, single or differential in to differential out SAR interface.
Emerging precision wideband FDAs (1) provide many useful features in driving 16- to 20-bit differential input SAR ADCs. FDAs intrinsically look like differential inverting op amps with an added output common mode control loop. They always present a relatively low input resistor that must be driven. Commonly, designs will buffer an unknown source impedance with a standard precision op amp using it to drive the FDA input resistor. A relatively complete design example is shown in Figure 1. This will be modified to reduce the loading on the OPA376 (2) outputs while retaining the same design targets.
For this example, a SAR Vref = 4.5V has been assumed. This then requires a 9Vp-p differential signal for full scale where placing that around a 2.25V common mode voltage is easily achieved using the Vcom pin of the THS4551 FDA.
Some of the features in this initial example design include:
- The inputs include overdrive protection to ground and the +3.3V supply used for the OPA376 buffers. Those BAV99 diodes are current limited by series 499Ω resistors which then also form a 200kHz bandlimiting pole at the op amp inputs. Using a lower supply on the input buffers guarantees the THS4551 cannot be overdriven.
- Like many RR I/O op amps, the OPA376 has more linearity on the output than input pins. Here, a full scale 0 to 2V input range is assumed to stay below the input stage crossover at 1V below the 3.3V supply – retaining the best input offset voltage. Setting a gain of 1.5V/V, the OPA376 outputs provide a full 0 → 3V swing with good headroom using the -0.23V negative supply (3) and 3.3V positive supply. At the maximum 3V output, those feedback and gain resistors require 1mA of output current back to ground.
- To take the full 6Vp-p differential swing available at the outputs of these two input buffers, the FDA is configured for a gain just slightly below 1.5X to deliver a maximum differential swing of 8.98Vp-p to the ADC. This would occur for the two inputs swinging 180o out of phase 0 to 2V.
- The FDA feedback resistors are selected very near the recommended 1kΩ value. This required the input resistor (what the OPA376 must drive) to be 681Ω. With the upper channel input at 2V and the lower at 0V, the FDA summing junctions are at 1.8V forcing the lower OPA376 to sink 1.8V/681 = 2.6mA load current.
- The output RC network is very typical of high resolution SAR ADC requirements. The 2.2nF differential capacitor and 24.9ohm isolating resistors form another 1.45MHz pole.
The relatively low 681Ω load resistor for the OPA376 (or dual OPA2376) may be too low for the desired harmonic distortion using this low power input stage. Also, the max load currents will slightly reduce their available linear output swing. One easy option is to simply scale the FDA resistors up in the same ratio. This is often a good solution but here the FDA resistors are selected to both add negligibly to the total FDA noise contribution and retain good phase margin. Simply doubling them will impair both - but might still be acceptable for many applications. The option considered here is to implement a partially active output impedance which will also have the effect of raising the input resistor that needs to be driven by the precision input buffers. While the OPA376 is an excellent choice for a low noise and power precision input stage, Table 1 shows a few similar alternates devices.
Alternate choices to the OPA376 input buffer.
Similarly, while the THS4551 is a very good SAR driver selection, Table 2 shows some alternate selections.
Alternate choices to the THS4551 precision FDA