modified 9 years ago

Hp34401 hi-G Input offset servo

00:43:03

In HP 34401 service manual (page~156) you could see this structure in u101 chip near input. Maybe a digital DC servo? Or digital temp. compensation or both... 

Pulse generator replace PRE_ADJ signal, which seems to be a PWM.. 

published 9 years ago

hurz

9 years ago

If you talk about a multimeter 6 1/2 digits. What google found is u101 is a 100:1 voltage divider

adrx

9 years ago

You talking about U102 which is a resistor "asic" implementing various presicion resistor for voltage and ohm current source. I m talking about u101, an "asic" with hi linearity mosfet switches and an integrated amplifier. It could be a nice chopper amp used with the dual matched fet following (u104+u106+bias circuitry). maybe using the pwm to feedback from AD converter

adrx

9 years ago

Maybe not a chopper amp but a digital chopper stabilizer for the jfet amplifier...

hurz

9 years ago

And what is the servo for?

adrx

9 years ago

To null amplifier&ADC offset. Because I can't understand the purpose of the pwm in a normal amplifier structure, I m thinking it's a chopper amplifier with a dc servo through pwm calculated from adc input. Each 1/N adc read, input is shorted then the adc output code is used to feed the pwm timer. Maybe pwm is only used when input are shorted to look for the null point, then digitaly substract the offset from real reading.

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