Using ATE to test digital programmable precision resistors for power supply loads

The digitally programmable precision resistor shown in Figure 1 can be used as a microprocessor-driven power load in a custom-designed ATE (automatic test equipment). IC1 is an 8-bit current-output DAC, a DAC08 type DAC, which drives the current-to-voltage converter IC2A, which in turn drives the gate of the power MOSFET Q1. The device under test is connected to J1 and J2. In operation, the current from the device under test forms a voltage across the sampling resistors R8A and R8B. Amplifier IC2B drives the reference input of IC1 and closes the feedback path. When the voltage drop across R8A and R8B reaches the VBE(ON) of Q2, transistor Q2 shunts the gate drive current of Q1 to provide overcurrent protection. VO and IO are the output voltage and output current, respectively, N represents the equivalent decimal value of the binary input applied to IC1, and A is the gain of the amplifier stage IC2B. R1 is composed of R1A and R1B in parallel. Equation 1 describes the load current of the circuit:

Solving Equation 2 gives the output resistance of the circuit:

Using the component values ​​shown, the equivalent resistance range of the circuit is from about 5.5Ω for all "0" binary inputs (i.e., N="0") to 255Ω for all "1" binary inputs (i.e., N = 255).

You can modify the component values ​​of the circuit to cover other resistance ranges. Replacing the 8-bit DAC08 with a 10-bit D/A converter can improve the resistance accuracy. To increase the power handling capability of the circuit, Q1 can be replaced with a higher power MOSFET and a suitably sized heat sink. Capacitors C3 and C4 control the bandwidth of the circuit.