SLASEL0B October 2019 – June 2020 DAC11001A , DAC81001 , DAC91001
PRODUCTION DATA.
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 G_{V} = 1, as shown in Equation 3. R_{1} and R_{2} 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 R_{1} = R_{2}, R_{SENSE}x G_{I} must be 500. Choose G_{I} as 50 so that R_{SENSE} can be 10-Ω. For a ±20-mA output current, the voltage drop across R_{SENSE} is ±200-mV. In case the design requires a lower voltage headroom, choose a higher value for G_{I} and a smaller resistance value for R_{SENSE}.
There is no equation to select C_{1} and C_{2}. The values of C_{1} and C_{2} 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 C_{1} = C_{2} = 100 pF. The 1-MΩ resistors in the circuit are used for making sure the amplifiers are not left in an open-loop state.