DIY precision voltage follower with Giga ohm input impedance ?

[BravoV]

After watching Dave's EEVblog #584 - What Effect Does Your Multimeter Input Impedance Have ?, just curious how difficult or the challenges if we want to build an add on box for the DMM front end that has Giga ohm impedance ? Just for low DC voltage measurement.

Its like uCurrent, a separated box for current measurement to complement the DMM, but this time for low DC voltage and has very high input impedance.

Assuming we skip the input protections and other safety measures as in normal DMM does, is it difficult ?

As novice, I'm thinking of a battery based circuit with an fet input, low offset and precision op-amp configured as voltage follower, is this enough ?

Appreciate some thought from experienced fellows.

[sync]

It's easy. I just breadboarded(!) this circuit. My measurements:
Offset voltage (shorted input): <10uV
Error from -3.5V .. +3.5V: <10uV
Input impedance @ -3.5V: >350Gohm
Input impedance @ 3.0V: >300Gohm
Input impedance @ +3.5V: >20Gohm
Bias current @ 0V: <30pA

For reliable high impedance I would mount the input path free standing in the air. A problem is that the output is not protected.

[David Hess]

It is not difficult to do but there are some features besides robust overload performance that I might include:

1. Suspend the supply voltage to the circuit to increase the common mode input voltage range by a lot.
2. Use a low noise FET with offset correction provided by a chopper stabilized amplifier to lower the broadband and flicker noise.

I do not know about currently produced handheld meters but the Fluke 8060A handheld multimeter supports bypassing the 10 megohm input divider to provide > gigohm level input resistance on the 200mV and 2V ranges.  My best bench meter, an HP3478A, of course always operates this way on the low ranges which is annoying when a standard 10 megohm input is needed as I discovered once.  My Tektronix DM502 3.5 digit multimeter can be jumpered to do this while my higher resolution DM501 4.5 digit multimeter cannot.