Ma, Μa And Adc; Active Filter; 800 Hz Low Pass Filter; Rms To Dc Converter - Fluke 87 V/AN Service Manual

87 V/AN
Service Manual
scaled voltage at ACAMP_O is also routed within U1 to comparators that will produce a
digital signal at COMP_O (U1 pin 24) ready for frequency measurement.
mA, µA and ADC
The mA/µA input and the A input are connected to R4 (U1 pin 10) as described in Table
11. The voltage at R4 is proportional to the product of the current being measured and
the shunt resistance that is in use. The voltage at R4 is routed by U1 internal switches,
filtered, buffered with a gain of 1 or 10 depending upon the range selected and passed to
FE_O (U1 pin 19) ready for measurement. If the Hz function is selected, the signal at
FE_O is also routed within U1 to comparators that will produce a digital signal at
COMP_O (U1 pin 24) ready for frequency or duty-cycle measurement.
The U1 buffer amplifier connected to FE_O (U1 pin 19) is zeroed digitally by the
microprocessor. The voltage at ZERO_IN (U1 pin 14) is routed by U1, buffered with the
selected gain and passed on to FE_O ready for measurement.

Active Filter

C9, C10, R17 & R18 are used in conjunction with an operational amplifier internal to U1
to form a low-pass active filter. This configuration produces the equivalent of two
cascaded filters, each at approximately 6 Hz. R18 is bypassed by switches in U1 when
the input impedance of the range voltage divider is above 1 MΩ.

800 Hz Low Pass Filter

R86, R87, R88, C36, C37, C38 & U4 form an 800-Hz low-pass filter (three pole
Butterworth) that can be selected in the VAC function. This filter is used to stop high
frequency noise such as that encountered in motor drive controllers from passing on to
the measurement circuits. The A switch of multiplexer U12 is used to select between
either the input or output of this filter as an input to the RMS Converter. The B switch of
multiplexer U12 can select the output of this filter for input to CONV_IN (U1 pin 13)
and bypass the RMS Converter when the Frequency function or pm option is
selected.

RMS to DC Converter

C6, U7 & C7 form the RMS-to-DC conversion circuit. C33 & C35 are RF-bypass
capacitors. C32 & C34 are power-supply bypass capacitors for U7. R29, R30, R32, R33,
Q6 & Q7 form the power-control circuit for U7.
The selected AC signal at the A output of U12 is passed to U7 pin 1 via DC-blocking
capacitor C6. U7, with the aid of the averaging capacitor C7 on pin 5, produces a DC
output at pin 6 that is proportional to the RMS of the input.
U7 is powered on and off as needed for the selected Meter function by the
microprocessor controlling the AC* signal. When AC* is driven to a logic high (+0.8 V),
Q6 is turned on, turning on Q7, which connects the positive end of the battery (6 V
minimum) to the pin 3 of U7, thereby powering it down. When AC* is driven to a logic
low (-2.5 V), transistor Q6 is turned off, turning off Q7, which allows U7 pin 3 to be
pulled to +0.8 V through R33, thereby allowing Q7 to power on.
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