*Albert O’Grady*

*EDN*

Figure 1 shows a configuration that implements a digitally programmable resistor using a quad op amp and a multiplying DAC. The circuit is equivalent to a voltage-controlled resistor. The simulated resistor has a value that reflects the ratio of a fixed resistor (R_{X}) and a control voltage. Applications include generating precise resistance values for remotely controlling monostable multivibrators and configuring voltage-controlled loads in simulation circuits. The circuit provides linear control of resistance using the AD7538 14-bit multiplying DAC. You can obtain logarithmic control of the resistance by using an AD7111A logarithmic DAC as the voltage-control element.

Figure 1. |
A multiplying DAC implements a linear, digitally programmable resistor. |

Analysis of the circuit in Figure 1 reveals the following:

where D is input code to the multiplying DAC.

If R_{1} = R_{2} = R_{3}, then

and

The circuit in Figure 1 operates as a voltage-controlled current source. You can adapt it for use as a basic functional block in the design of a biquad filter. In the adaptation, you modify the circuit to provide a voltage-controlled capacitor rather than a resistor.

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