Would LT1008 also have been a good choice in the past, as the main difference (to LT1012) seems to be internal/external compensation?
The LT1008 is a fine choice and in some cases better because the external compensation allows its dynamic performance to be optimized. Before I found the OPA140, I was going to use the LT1008 to replace the damaged input buffer on my Tektronix 4.5 digit DM501A multimeter.
Because of the flawed design of the ADC in the DM501A, the input buffer must have high CMRR so even the best older precision JFET operational amplifiers are barely good enough unless tested and selected for high CMRR.
I´ve looked at the input amplifiers of a few other DMM´s, and found a few interesting OP´s that could be used:
AD795 (from the HM8012, 1-2pA Ib but -100µV Vos and 1-3µVpp Noise)
AD706 (from the 34401A, 50-200pA Ib and +30µV Vos and 0.5µVpp Noise)
In the past if a low input bias current bipolar part was not used, then the highest precision JFET input part was. The AD542 (25pA Ib, 250uV Vos, 1uV/C Vos Drift, <100uV/V CMRR, and 2uVpp Noise) was an early precision BIFET part recommended for this application. I am sure Burr-Brown had a bunch. Another recommended low input bias current bipolar part was the National LM11 (25pA Ib, 100uV Vos, 1 uV/C Drift, 0.3uV/V CMRR, >5uVpp Noise) which is an improved 308. In the past I used a lot of LM11s.
The limit to performance with JFET parts was usually their common mode rejection ratio. I assume that high resolution meters which used them without an automatic zero implementation like the ICL7106 bootstrapped them?
An Idea that I had was to attach a Peltier Element to the Input Amps and alter their temperature to the point, that their offset voltage is nearly zero.
The primary problem with that, besides the thermal EMF caused by the leads, would be that the offset drift and the offset are rarely of the same polarity.
They used to make constant temperature oven "hats" for the metal TO-99 package.
Bipolar precision operational amplifiers since before the OP-07 are designed so that the offset null adjustment also minimizes offset voltage drift. I do not know of any JFET parts with this property but if the temperature is regulated, then the offset null adjustment can be used to trim the offset.
For an DMM input amplifier (unless using an old chip like ICL7106) one would not care about the absolute offset voltage, as this can be subtracted in software anyway. So critical parameters are more like the input bias current (usuallaly < 50 pA are aimed for, ideally < 10 pA) and the voltage dirft.
The ICL7106 has its buffer built in with offset corrected as part of its automatic zero process, but there were variations of that idea intended to be used with an external buffer.
except in low-cost applications it is never a good idea to use the input amplifier as the ADC driver. If one is already concerned about the current noise a zero-drift amplifier produces, the noise injected by the ADC is orders of manitude higher than that - considering the internal buffers are bypassed for accuracy reasons.
An opamp output appears as an inductor to a signal (or noise-) source attached to the output. That means the noise will be pushed into the signal source only slightly attanuated at best.
The only proper way to mitigate these issues is to have a dedicated ADC driver and appropriate filtering in between the amps.
This problem gets even worse if the input amplifier has a gain (much) greater than A=1.
I am not sure what you are saying here. Some precision operational amplifiers will have problems driving against the charge injection of the ADC which requires fast settling time but that will not show up at their input. It is easily solved by adding a low impedance output buffer *within the feedback loop* and capacitive decoupling of the signal at the ADC input.
Using a JFET amplifier (like OPA140 as a modern chip) and seprate switching the input is an option, but also not that easy. There are similar or possibly even worse switching spikes from doing the choper type offset correction under software control.
Potentially it is no worse than the ICL7106. I have been working on this problem for a while and suitable input multiplexers are few. Did any production multimeters use an input multiplexer made from discrete transistors like the 3N series small signal MOSFETs, or even JFETs?