There are more than 200 lines and every line carries voltages in the range of 0-36V.What are all the possible ways to convert 0-36V (pwm signal,5kHz) to 0-5V for ADC input?
Note: I should not use voltage divider directly on the line.
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1\$\begingroup\$ Why can't you use a voltage divider? It's the obvious and simplest solution. \$\endgroup\$– Nick JohnsonCommented Nov 6, 2015 at 14:25
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\$\begingroup\$ Yup, but on the other end of the supply line, DUT is placed, which will misunderstand the voltage divider as load. Only very few micro amps allowed in the line. \$\endgroup\$– zuccy1Commented Nov 6, 2015 at 14:38
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\$\begingroup\$ Can't you use a very stiff voltage divider, or buffer the line, then? You should edit your question with the actual constraints you have. \$\endgroup\$– Nick JohnsonCommented Nov 6, 2015 at 15:10
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\$\begingroup\$ @NickJohnson, Yup, i can use a buffer and a divider, that could also solves my problem. \$\endgroup\$– zuccy1Commented Nov 9, 2015 at 15:09
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1 Answer
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If you don't want a continuous current draw (and an average that is only in the region of a few microamps) then consider using a low leakage analogue switch to sample the line voltage and charge a small capacitor up via a resistor. The resistor and capacitor form a potential divider and the capacitor voltage charges up to a voltage that is dependant on line voltage, resistance of the resistor, capacitance and length of time the switch closes for.
After a measurement is made, the capacitor can be discharged by a MOSFET or another analogue switch and the next line measured in the same way by using the analogue switch connected to its line: -
simulate this circuit – Schematic created using CircuitLab
The rate that the capacitor charges is exponential and if the line voltage is (say) 30V and CR is 1n x 1M (= 1msec) then you'll want to keep the line sense switch closed for only a fraction of that time to ensure that the ADC input does not hit the 5V limit imposed by the zener protection diode.
I calculate that for 3V across the capacitor. the switch needs to be activated for 105.4 usecs (see this calculator).
Of course you may be able to get away with a simple potential divider in place of the R and C above but your question is a bit unclear about what is and isn't acceptable. Using a potential divider that scans all the inputs via analogue switches should be OK I believe in your application.
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\$\begingroup\$ Thanks. it is really a good idea, as you said i gonna use low leakage switch at the front to switch between the node lines(16x1 or 32x1), but then the frequency at RC bridge becomes very high. As my input voltage range is known, i'm thinking of using different Capacitors (select thro Demux) for different voltage ranges for a better precision output values. \$\endgroup\$– zuccy1Commented Nov 9, 2015 at 15:07