Single-ended to differential amplifier at ppm level - Questions on DN1032

Hello everyone,

It's been a while since I'm doing research on ADC acquisition close to 10 ppm level.

For the short story:
I'm currently design a voltmeter with high precision.
I have almost all my acquisition chain in my architecture, except one:  the single-ended to differential amplifier (needed for the ADC).
I'm twisting my neurons about this stage which can fly away the linearity of the ADC.

I found the example from the datasheet of the LTC2442 (p.28) and the Design Note 1032 (DN1032) which explains and answers to my problematic by a similar solution: create the complementary signal with an inverting amplifier (and centering it, in passing).

However, I still have 2 interrogation on the given schematics (I am refering to the DN1032):

Question 1
The inverter amplifier which creates the -180° phase signal has a gain fixed by 2 resistors R5 and R6 (more the additional resistor R4).
There is no mention anywhere of the effect of the drift of this gain.
For me, it will impact directly the negative signal and imbalance the input of the ADC. Moreover, I don't see how this gain can be calibrated.
The DN-1032 explains that a quad matched resistor is needed to divided the signal precisely after the amplifier, but nothing for the gain of the inverter.
Note: The centering of the differential signal has less impact on the result of the ADC. So, I don't talk about the resistors R1 and R2. 


Question 2
The inverting amplifier creates a shift phase (a delay) compare to the direct positive signal.
For the very low frequency, it will be good but for higher frequencies (>10kHz), it's can be a problem.
On solution can be to minimize the delay with an active compensation into the feedback of the inverting amplifier.
Another solution will be to use a fully differential amplifier which by design ensure a symmetry but they are often design to AC and less concern about DC.
In the book "The Art of Electronic", it shows different ways to create a differential stage but they used a lot of a resistor (more than a quad which are not available in network).
However, I wonder if the fact to choose a truly differential stage by design will improve the precision because it wills add more components and with that more problems...
 What will be your point of view or your approach?


More details on my project:

For now, I have in mind to use for the main ADC: LTC2442 (or LTC2376-20). Principally for their linearity and their stability.
I expect 18 bits absolute precision after calibration and for low frequency (<1kHz).
In front-end, I will use Jfet famplifiers op-amp (ADA4625) or, if I success to buy it, a dual jfet monolithic to build a discrete Jfet composite amplifier.


Thank you for your reading, Best regards,