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
■ FIGURE 3. The input