Finally, some requirements. As DC noted, some (very few) optocouplers transfer measurable amounts of energy from the input to the output. Most do not, they use energy from the input to control energy externally applied to the output. For your application, a better approach would be a charge pump, a type of switched-capacitor DC/DC converter. What is the output current requirement?Sorry, I forgot to mention that I need variable voltage between -1V to 1V. for 0-1V I am using a PWM to vary the voltage. Now I need a way to also have -1V to 0V.
You could roll your own switched capacitor network...Charge pumps do seem to be a good option for me. I looked at TI and Maxim for the their inverting charge pumps and they do not offer anything in the range of 0 to 1V. The lowest Vin they allow is 1.5V which corresponds to -1.5V out. Do you suggest any vendors?
Below is the LTspice simulation of a simple negative charge pump driven by a 0 to +3V PMW signal.
The output is shown with a 10kHz PWM frequency for duty-cycles of 90%, 75% and 57%.
Note that the negative output voltage is inversely proportional to the (positive) PWM duty-cycle.
An output of -1V is achieved with about a 57% duty-cycle (rather than the theoretical 66%) due to the offset from the diode.
[Edit: Changing D1 to a Schottky type diode will reduce this offset and make the output closer to the theoretical duty-cycle value as shown in the second simulation below where 65% duty-cycle gives a -1.01V output.]
The capacitor and resistor values are determined by the PWM frequency, how much ripple you can tolerate, and the required response time of the circuit.
View attachment 93649
View attachment 93652
The op amp requires a negative supply voltage to output a negative voltage. It doesn't generate that on its own.Great, thank you very much. I guess another option is to just use an inverting opamp with gain of -1. Is there an advantage of using a charge pump over inverting amplifiers?
The world has changed. This search page shows over 4,000 op-amps that will operate on 3V or less.you would need an op amp that will operate on ±1.5V which are likely somewhat rare.
Dang. Guess I'm behind the times.The world has changed. This search page shows over 4,000 op-amps that will operate on 3V or less.
Well, I did guess a bit...Where did you see that info in the data sheet?
Okay. I see what your are referring to.Well, I did guess a bit...
The open circuit voltage is 7 volts minimum and the short circuit current is 12 ua minimum. The graph at the top left of page 5 is inscrutable to me in that it does not seem to reflect the open and short circuit values in the Coupled Electrical Characteristics table on page 3.
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by Wes Brodsky
by Luke James