Digitally programmable resistor serves as test load

Figure 1 illustrates a digitally programmable precision resistance that can serve as a microprocessor-driven power-supply load in custom-designed ATE (automatic-test equipment). An 8-bit current-output DAC, IC₁, a DAC08, drives current-to-voltage converter IC_2A, which in turn drives the gate of power MOSFET Q₁. The device under test connects to J₁ and J₂. In operation, current from the device under test develops a voltage across sampling resistors R_8A and R_8B. Amplifier IC_2B drives IC₁'s reference input and closes the feedback path. Transistor Q₂ provides overcurrent protection by diverting gate drive from Q₁ when the voltage drop across R_8A and R_8B reaches Q₂'s V_BE(ON). V_O and I_O represent the output voltage and current, respectively; N represents the decimal equivalent of the binary input applied to IC₁; and A represents the gain of the amplifier stage IC_2B. R₁ comprises the parallel combination of R_1A and R_1B. Equation 1 describes the circuit's load current:

  EQUATION 1

Solving Equation 2 yields the circuit's output resistance:

  EQUATION 2

You can modify circuit values to cover other resistance ranges. Replacing the 8-bit DAC08 with a 10-bit D/A converter increases resistance resolution. To increase the circuit's power-handling capability, replace Q₁ with a higher power MOSFET and an appropriately sized heat dissipator. Capacitors C₃ and C₄ control the circuit's bandwidth.

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