The DAC11001A is an excellent choice for this application to meet the 20-bit resolution requirement. Switch SW is used to toggle between force-voltage and force-current modes, as shown in Figure 56. The OPA828 is a high-precision amplifier that provides a good balance between dc and ac performance, and can supply ±30-mA output current. The INA188 is a zero-drift instrumentation amplifier with gain selected with an external resistor. The external resistor is not shown in the drawing for simplicity. The gain resistor is not required for a gain of 1. Equation 2 shows the calculation of the voltage gain when switch SW is in position 1.
Precision reference sources are available at 5 V or less. Use a ±5-V reference with a 2x gain configuration to get an output of ±10 V. The DAC output amplifier sets the gain at 2, assuming GV = 1, as shown in Equation 3. R1 and R2 are 1-kΩ each. Equation 3 shows the calculation for the current gain when the switch is in the position 2.
In order to get ±20-mA output current range with R1 = R2, RSENSEx GI must be 500. Choose GI as 50 so that RSENSE can be 10-Ω. For a ±20-mA output current, the voltage drop across RSENSE is ±200-mV. In case the design requires a lower voltage headroom, choose a higher value for GI and a smaller resistance value for RSENSE.
There is no equation to select C1 and C2. The values of C1 and C2 depend on the stability criteria of the reference buffers when driving the reference inputs of DACx1001. The values are obtained through simulation. For the OPA828, use C1 = C2 = 100 pF. The 1-MΩ resistors in the circuit are used for making sure the amplifiers are not left in an open-loop state.