Gain Of 1 - HP 98640A Installation And Reference Manual

98640A Analog Input Interface
WARNING
THE ANALOG MUX SHORTS EACH INPUT TO
GROUND THROUGH A lkOHM RESISTOR WHEN
POWER IS NOT APPLIED.
Programmable Gain Amplifier (PGA)
The PGA consists of two op amps (U44 and US4») an analog mux (U45) and a custom precision resistor
network (U24). Figure 3-3 is a diagram of the PGA) and figure 3-4 shows the innards of the preci-
sion resistor network.
The PGA selects the gain for a reading by selecting the appropriate string of resistors from the net-
work in U24. (This selection is accomplished by the analog mux) based on the values of address lines
AA4 and AAS.) The resistor string thus selected becomes part of the feedback circuitry for the op
amps.
An op amp)s goal in life is to have its + input at the same voltage as its - input. When an op amp)s
output voltage is fed back into one of its inputs) the op amp can approach its goal (equal voltages at
the inputs) by varying its output in some suitable manner. In the case of the PGA) the differential in-
put voltages are brought into the + inputs of the two op amps. The output of each op amp is fed back
into its - input.
GAIN OF 1
For a gain of 1) the operation of the feedback circuit for a single op amp is simple:
1)
The input voltage comes in at the + input.
2) The op amp produces an equal output voltage.
3) That equal output voltage is fed directly back to the - input.
4) The
01'
amp detects that the voltages at the two inputs are equal) and leaves things as they are.
The resulting stable output voltage is available for use by the next stage of the analog circuitry.
Thus) for a gain of 1) the two op amps simply pass their input voltages on to the next stage.
3-7