Analog front-end for the voltmeter (3/3)

In the last part for the analog front-end I tried to make all in one simulation for my first prototype:

The voltage offset of the LT1167 input op amp is automatically trimmed with 3 additional op amps. The first one is used for an input buffer, the second one subtract output voltage from the input one and the third one is used as buffer to the VRef pin of the LT1167 which requires low resistance. Simulation results shows that a small resistor (R6) must be put in serial to R5 in order to minimize the offset.

The next stage is the automatic voltage range. One SPDT relay can be used to switch between the middle point of the voltage divider (1:10 ratio, range from 1.25 to 12V) or directly from the LT1167 output (1:1 ratio, range from 0-1.25V). The window comparator for triggering the voltage range switch is realized with 2xLT1011 comparators and uses 2 voltage references : +/- 1.25V. In the simulation is not used hysteresis settings for the comparators, but special attention should be taken for accurate switching.

The last stage is the level shifting. The input signal is shifted by 0.625V, taken after the 1:2 divider from Vref source. The output of the stage have additional 1:2 voltage divider. 

All resistors will be from LT5400 quad matches series, which have matched long term drift of 2ppm. 

Here are examples for calculation of unknown input voltage when the output voltage is read out from the ADC using the simulation results:

Vin = (Vshift - Vout) * 2 when voltage range is 0-1.25V and input signal is positive:

Vout = 0.62499988 V 

Calculated Vin = 0.0000024V

Simulation Vin = 0V, error is 2.4 ppm

Vout = 0.12500075V

Calculated Vin = 0.9999985V

Simulation Vin = 1V, error is 1.5 ppm

Vin = (Vshift - Vout) * 10* 2 when voltage range is 1.25-12.0V and input signal is positive:

Vout = 0.024990104V

Calculated Vin = 12.00019792V

Simulation Vin = 12V, error is 16.49 ppm

Vin = -(Vout - Vshift) * 2 when voltage range is 0-1.25V and input signal is negative:

Vout = 1.124999 V 

Calculated Vin = -0.999998V

Simulation Vin = -1V, error is 2 ppm

Vin = -(Vout - Vshift) * 10 * 2 when voltage range is 1.25-12V and input signal is negative:

Vout = 1.2250097 V 

Calculated Vin = -12.000194V

Simulation Vin = -12V, error is 16.16 ppm

When the Vout is less then the Vshift, the input voltage is positive and when Vout is greater then Vshift, the input voltage is negative.