Fluke 5790A Service Manual page 70

5790A
Service Manual
2-92. Instrumentation Amplifier
The instrumentation amplifier amplifies the difference between the output of the Null
DAC and the output of the FTS circuit on the Transfer Assembly. The output of the
instrumentation amplifier is fed directly into the A/D Amplifier IC, U24. The result of
this system is greatly improved resolution from the A/D IC. The instrumentation
amplifier is switched between a unity gain and an X10 configuration.
Components U14 and U15 are used as 4-to-1 multiplexers to select the input signal to the
noninverting and inverting inputs to the instrumentation amp. The noninverting input can
be connected to lines RCL, DAC-SNS-HI, R-COM or NULL-DAC. The inverting input
can be connected to DIV-OUT, DAC-SNS-HI, R-COM or NULL-DAC.
Components U19 and U20 buffer the inputs. Components U21, Z2 and Z3 convert the
differential input signal to a single-ended output, available at TP14. (Equivalent to TP10
minus TP9). The single-ended output is fed to the A/D IC through R27. If this voltage is
too large for the A/D to read, a signal six times smaller is available through R29 and R30.
Maximum resolution is obtained using U22, R24, and R25 as an X10 amplifier.
Capacitor C14 filters the high frequency noise, and VR2 and VR3 clamp the output to
less than the A/D ICs power rails. Components U2, R45 through 47 and C42 through 44
form a three-pole Bessel active filter to reduce noise and ripple.
2-93. Switchable Active Low-Pass Filter
A five-pole Bessel active filter attenuates low frequency ripple. U24 pin 58 is the input to
the filter and pin 56 is the output. Pins 59 and 60 connect to an internal op amp. This op
amp together with U3, C1, 2, 18, 19, 23, R23, 38, 33, 40, and 41 comprise the filter.
Switches inside the A/D IC can precharge C18 and 19 quickly to reduce the settling time.
2-94. A/D Converter
A proprietary Fluke IC (U24) containing an A/D converter and frequency Counter forms
the basis for this circuit. Voltage is measured through pins 3, 14, 15, 16, 18, or 23. The
following list describes the U24 pin functions:
2-40
Pin 14 is the diagnostic input.
Pin 15 is the divide-by-6 input. It measures larger voltages than the unity-gain
and X10 inputs, but with the least resolution.
Pin 18 is the unity-gain input. It measures voltages up to 3 V with less resolution
than the X10 input, but more resolution than the divide-by-6 input.
Pin 23 is the X10 input. It measures voltage up to 32 m.
Pins 28 through 35 comprise an output port that control the inputs to the
instrumentation amp through U14 and U15.
Pins 36 and 37 connect to Y1, C15, and C16 to provide the clock for A/D timing
and serial interface communication.
Pins 39 and 40 are the serial interface, buffered by U25 pins 1 through 4.
Pins 45 through 50, together with C17 and Z1 pins (4 through 8) form the
integrator for the dual-slope A/D Converter.
Pins 51 and 52 connect to a 1 V reference as described under "Null DAC."