OK, I added TC4427 dual non inverting drivers for driving LEDs.
I decided to put ca. 30mA through single LED.
Measurement using commercial IVD.
series capacitor of the demodulator was increased to 2uF
for VLED=16V
set 0.0 measured 0.000000V
set 0.1 measured 0.999988V
set 0.2 measured 1.99997V
set 0.3 measured 2.99996V
set 0.4 measured 3.99996V
set 0.5 measured 4.99997V
set 0.6 measured 5.99998V
set 0.7 measured 6.99999V
set 0.8 measured 7.99999V
set 0.9 measured 9.00000V (voltage calibrator was used to provide exact value)
It is slightly better.
The reading changes much less when I change VLED from 16V to 12V.
This must be a result of flat resistance vs. LED current curve in this region.
BTW--- The input bias current of the OP77 is around 1.2nA at room temperature... I didn't bother with the math, but I wonder if this is enough to cause this problem?
Yesterday I changed opamp to MAX430 with typical 30pA input current. There was no significant difference between OP77.
@DiligentMinds.com
Slope compensation question is very interesting, but It might be difficult to build this and prevent it from oscillating at least in this simple version. Do You have any example to show this slow and fast opamp solution ?
I wonder if it makes sense to continue this very simple, single +15V supply solution -or- just to try to copy DATRON 1281 circuit.
As I understand, they used mid tapped transformer on primary and secondary. This makes possible to do this slope compensation fairly easy with one opamp. However they used loop filter with quite high time constant (dual RC circuit with Tau=1ms). I wonder what frequency they use for modulation. I wouldn't be surprised if they used something like 10...20Hz.