DC amplifier analysis and redesign.

I am trying to understand an old meter and to propose similar with to today's components.
With the majority of the problem I have counseled, but at the amplifier more than fumble


Few words of introduction Amplifier is substantially
DC amplifier with Gain 1,10,100 (switching resistor in RSW1),
input voltage 0- +-10V output voltage max +-10V
amplifier work as amplifier for 24bit ADC

Actually I'm not too clear as the amplifier operates and why it was designed just like this.
I understand, dual FET input is there because low Input Bias Current in order pA.
I do not understand all the rear part U153 with NPN.

I welcome any evidence on the functioning and benefits of the current solution
I particularly welcome the idea of how to deal with similarly good amplifier with today's component base.

Hi,

DC amplifier for a 24 bit ADC...
With a range of +/-10 V one LSB is about 1.2uV. I doubt that the input stage with the FET pair is near this value regarding offset voltage and offset voltage drift.

But I don´t know this device.

I assume a chopper stabilized amplifier is more suitable.

Klaus

You're right.
But surely you know that there is noise, temperature drift and so forth
and also a technique to suppress them, zeroing, calibration, mathematical methods etc.
But things.
The device is actually chemical analyzer, but technical is it 6.5 Digit precision voltmeter designed Agilent. The accuracy of the instrument
is on input voltage +100mV i.e amplifier gain 100x, max error in 24 hour measurement is +- 300uV (max error it is error in input stage, error in amplifier and error on ADC)
Whoever designed the amplifier was certainly a top expert suggested it as suggested.
Design is maybe 20-25 years old, but already existed LTC1100 Precision, Zero-Drift Instrumentation Amplifier. To design such a complex solution must be a reason why in seeking solutions to the "discrete" integrated with few external resistors.