The OPA1637 provides the advantages of a fully differential amplifier (FDA) configuration that offers very low noise and harmonic distortion in a single, low-power amplifier. The FDA is a flexible device, where the main aim is to provide a purely differential output signal centered on a user-configurable, common-mode voltage that is usually matched to the input common-mode voltage required by an analog-to-digital converter (ADC) or class-D amplifier. The circuit used for characterization of the differential-to-differential performance is seen in Figure 7-1.

Figure 7-1 Differential Source to a Differential Gain of a 1-V/V Test Circuit

A similar circuit is used for single-ended to differential measurements, as shown in Figure 7-2.

Figure 7-2 Single-Ended to Differential Gain of a 1-V/V Test Circuit

The FDA requires feedback resistor for both output pins to the input pins. These feedback resistors load the output differentially only if the input common-mode voltage is equal to the output common-mode voltage set by VOCM. When VOCM differs from the input common-mode range, the feedback resistors create single-ended loading. The characterization plots fix the R_F (R_F1 = R_F2) value at 2 kΩ, unless otherwise noted. This value can be adjusted to match the system design parameters with the following considerations in mind:

• The current needed to drive R_F from the peak output voltage to the input common-mode voltage adds to the overall output load current. If the total load current (current through R_F + current through R_L) exceeds the current limit conditions, the device enters a current limit state, causing the output voltage to collapse.
• High feedback resistor values (R_F> 100 kΩ) interact with the amplifier input capacitance to create a zero in the feedback network. Compensation must be added to account for this potential source of instability; see the TI Precision Labs FDA Stability Training for guidance on designing an appropriate compensation network.