Datron Calibrator 7v to 10v conversion with a transformer

[Kleinstein]

I first was a little suspicious about the H11F1, as it is relatively slow and not that low in impedance, but it has two big advantages: no charge injection and it is independent of the DC level at the switch. Both could be a big problem with normal CMOS switches, and might require a bootstrapped power supply. During the time the switch is turned on or off the voltage at the switch is rather small (only residual from the slope) and thus not much effect on the switching time is expected. So I changed my mind and now like the H11F1 as a switch.

I don't think the inter winding capacitance should be that bad, as this would be an effect only for the time just after switching. The dead time before the signal is actually used should be large enough so that the capacitors should be well charged. This could be different if used for AC with a sine wave.

I would expect most of the slope to come from the coupling capacitors. So the split turn and separate drive like the Datron circuit might be good enough. If DC magnetization is a problem one might use a separate correction loop (or only adjustment pot), by checking for the 2 nd harmonic (or positive and negative peak values) in the driving current. For the secondary side the current solution looks quite good and might even be an advantage when using the split turn for driving, as it is not sensitive to uneven voltage for both halves.

Making things worse first to find weak spots is a good idea, especially a main cause of trouble is likely a combination of the slope from driving the transformer and non ideal demodulation that is sensitive to the slope. One source could be the nonlinear resistance of the H11F1, that sees about the voltage of the slope. So it might already help to have a resistor in the 1 K range in series from the transformer tap / capacitor, so that less voltage drop is at the switches itself.
If I understand the specs of the H11F1 right, nonlinearity can be up to 0.1% at 25 µA. This would be something like a 10 mV change from the slope, than could contribute something in the 10 µV range.

[doktor pyta]

Specially for Kleinstein I measured voltages with 1k resistor in series with both demodulator switches H11F1.

Measurement using commercial IVD.
series capacitor of the demodulator 2uF

for VLED=16V
set 0.0   measured 0.000000V
set 0.1   measured 0.999983V
set 0.2   measured 1.99997V
set 0.3   measured 2.99995V
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)

Comment: no visible change in results.


EDIT:
Before stopping this experiment I measured some parameters of the divider shown in Reply #2.
Single winding (10 turns): DC Resistance 0.17ohm, Inductance @120Hz: 20mH
Capacitance between adjacent windings: ca. 100pF

[doktor pyta]

In an old Polish book about metrology 'Etalony...'  I found information about inductive voltage dividers.
I took two pictures of it.

First chart shows 'relative internal current level vs. distance from the beginning of the winding for multifilar winding'. As I understand we are talking about current due to inter winding capacitances

Second picture shows how to deal with it: to connect calculated values of admitances (external capacitors).

Maybe the same error of ratio is present also in DATRON circuit, but as long as it is stable in time few ppm's of (constant) shift may not be so important.

[Kleinstein]

Thanks for doing the extra measurement with series resistors. So the slightly nonlinear resistance of the H11F1 seems to be not the culprit for the nonlinearity.

As the errors got larger with higher frequency, it might be a good idea to do future tests at a higher frequency. The H11F1 has a specified response time of 15 µs. Assuming at least 3 time constants, this would be about 50 µs or a 20 kHz limit for the clock to the 4017 and thus 2 kHz for the transformer.

For the effect of inter winding capacitance, it should be possible to test this, by adding capacitive loading and watch the change in voltage. Due to the nearly square waveform, I have some doubt that this is the problem. Anyway it might be interesting to know / check the output impedance of the transformer. I would expect something in the 1-10 Ohms range - so capacitors should charge quite fast.

Form the Datron circuit it looks like they use a frequency of slightly less than 1 kHz. But they also use faster switches and I don't think they want absolute accuracy, but just stable values. For just doing the 7 to 10 V step for a 10 V reference, there is no need to have an absolute value, just stable would be good enough. However understanding why the value is different from ideal can help to make it more stable and having something like accurate 1,2,....,10 V steps would be really nice.

Edit:
I just realized one source of error:

The capacitance in off state together with the now 2 µF capacitor make up a voltage divider that reduces the output voltage. The capacitance of the H11F1 is supposed to be in the 15 pF range (at 15 V), but it is expected to get larger at lower voltage. This error component should be independent of frequency and scale with the capacitance. The shape and size of the observed nonlinearity looks reasonably for this effect. One might be able to measure the H11F1 independently and than calculate the effect. The effect might be smaller with other (e.g CMOS) switches, but the principle problem should be the same.