Nuts and Volts - August 2008

Attenuator circuit (Figure 3) must perform: (1) give the input signal a fixed 2.5V offset; and (2) present an approximately 1 VRMS signal to the input of the ADC. The 2.5V offset is required because the op-amps and ADC are powered by a single supply (+5V) and will operate properly only with positive input signal voltages. This circuit allows the input voltage to be symmetrical around ground. Resistors R1 and R2 form a voltage divider that allows you to measure voltages greater than what can be sent through the op-amp circuits. The voltage applied to the input side of R5 needs to be symmetrical about ground and no larger than about 1 VRMS.

The circuit of IC1.1 gives the input signal the required 2.5V offset by configuring the op-amp as a “follower with gain.” The gain is two and is set by the formula: (R4/R3) + 1. The reason for the gain of two is that R5 and R6 essentially form a divide-by-two circuit. The resistors in this circuit (R3-R6) should not be modified. Neither should the voltage divider formed by R7 and R8 that develops the 2.5V offset. The loading of R5 and R6 on the 2.5V is negligible.

The circuits of IC1.2 and both halves of IC2 perform a

programmable amplification function. The three circuits are essentially identical except for the gain values. This description references the circuit of IC1.2. The values of the feedback resistors (R10 and R11) need to be determined based on your specific requirements. The gain of the circuit is RFB/R9 (RFB is the feedback resistor). You can calculate the resistor values based on the signals you want to measure and the gain required to get 1 VRMS. IC3 is an analog switch that allows you to select between two gain values for the stage by switching R11 in or shorting it. With the switch closed, the gain is 1 (0 db); with the switch open, the gain is 10 ( 20 db). The reason the switch for R11 (IC3) looks backwards as compared to IC4 and IC5 is that the board was easier to lay out with pins 1 and 2 reversed.

These resistor values and the relay allow for the following attenuation/gain values: attenuation = 28:1 (about -30db); gain = 0 db, 10 db, 20 db, 30 db, 40 db, and 50 db. You can also get attenuation values of - 10 db and - 20 db by using the attenuator and the appropriate gain values. I have elected to use 1% resistor values since