58452 VXIbus Universal Power Meter
3-2
Publication 21555, Rev. E, September 2002
3.3
Analog PC Board (A1)
The sensors are connected from the front panel through W3 and W2 which are connected to the
Analog Board through W2P1 and W2P2 for sensor 1 and W3P1 and W3P2 for sensor 2. The detected
DC voltage from the sensor is a differential voltage applied to pins 3 and 4 of J7 or J9. This differential
voltage goes to the U28 or U29 FET chopping circuit. The outputs are pins 8 and 9 of the chopper. An
incoming signal can either be fed straight through or inverted. The signal is fed straight through when
A1 and A0 are both low. It is fed through in the inverted mode when A1 is low and A0 is high. When
A0 is low and A1 is high the signal is shorted together and grounded, and when A0 and A1 are both
high, the input is open (floating). This provides chopper stabilized amplification when low power
signals are being received by switching the FET switch from the inverting to non-inverting mode and
back again at a rate of 300 times per second.
U31 is the 1st stage amplifier, and has programmable gain. The programmable gain is provided by U30
and the resistor ladder composed of R6 through 12 which will program gains of 1, 64, 512 and 4096.
U32A is the 2nd stage amplifier that provides programmed gains of 1, 8 and 64 using FET switch U33
and resistor ladder R16, R17 and R18. C167 (though shown on the schematic) is not loaded on the
board.
U32B/C/D and associated components provide a 6-pole bessel filter in the low pass configuration
-10 kHz with a 120
µ
s settling time. In normal operation, the signal will always go through this filter.
Other paths such as unfiltered, TP8, and 1st amp signals are provided for testing only and not used in
normal operation.
The 1st amplifier, U31, has an offset voltage injected into it via pin 7. This offset voltage comes from
the 12-bit DAC, U15. Offset voltages range from +5 V to -5 V which can effectively remove about a
±
1.2 mV offset at the detector. U34A&B are comparators which monitor the input signal for a too high
or too low condition. The too low comparator, U34B, will fire if the voltage is below 1 V, and the too
high comparator, U32A will fire if the voltage is above 9 V. This provides an A/D conversion range of 1
to 9 V. The too high and too low signals are used by the software to determine whether or not a range
change should be made. There are 7 ranges. Four ranges are processed by U31 (1st Amp), and three
ranges by U32A. Appropriate gains are set to keep voltages at the A/D conversion point between 1 V
and 9 V. R45 and R46, divide the 12 V regulated voltage to 1 V such that the too low comparator will
fire below 1 V to assert the too low signal. R36 and R37, divide the 12 V signal to 3 V, and R40 and R41
divide the incoming signal by 3 so that the too high comparator will always fire at 9 V.
Figure 3-2: Analog PC Assembly Block Diagram
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