I have a negative output voltage (-60V).
What i plan to do to regulate this voltage is to use a resistor divider at the output to receive a -2,4V signal instead of -60V.
Then, i want to compare this signal with a bandgap reference voltage source.
The problem is now, can i use a comparator to compare? If i use a standard bandgap, it only gives me a positive 2,4V right?
So, how can i sense a negative voltage?
You can follow your idea, simply using circuits working "under zero". If you can afford to load your -60V source by one milliamp, you can use an optocoupler. Or use an opamp to transfer the sensed voltage to a positive level.
Comparators with internal reference can be used simply "under zero", with their Vcc at GND and Vdd at say -5...-15 V. Their output, negative TTL or CMOS, can be used thru the optocoupler as above..
No.The opamp way is good but you must use the two-polarity power supply for it, +/- 5 or +/- 15 V DC. Then your DC input can be 0 to -5 (-15) V to operate correctly. You can use the opamp as a comparator, too.
You can use a positive bandgap reference of any voltage and your negative supply scaled to be equal and opposite. Then your inverting input is now 0V ,( with +Vin=0V) your (-) must now use the same source impedance with a common feedback gain.
You may add series RC values for derivative and proportional gain control and common RC feedback for integral control and now have a PID error control to optimize stability, overshoot and bandwidth for ideal step load response as you wish.
The feedback cap ought to have a series R for improved phase margin improvement but overall good noise reduction. You can also inject a soft start feedback.
I think the first thing to do is to convert the -60V into a positive change. So find a likely positive supply, say +5V. Now connect a 60 K resistor to the -60V line and a 5K resistor to the +5V line. the junction of them will be 0V when the -60v is present and +60/65 X 5V when its absent. So alter the resistors so that when the -60V is present the junction is +1V (61K + 4K). Now you have a change with in the voltage range of your comparators. It could be an idea to incorporate a clamping diode from the junction to earth (cathode to junction and anode to earth), just in case the -60 goes to -80V.
Frank[/QUOTE
Any OpAmp idea is pretty much obsolete, because my technology gives me only an opamp that works in a 0-5V range.
I thought about another simple idea (pic attached) where voltages are added at a node. When the voltage goes below -60V, the node is negative. The inverter gives a 5V output signal as a result. When the voltage lies above -60V, the inverter output is 0.
That is the theory. If i simulate it in Cadence with DC sweep, the node that should be -5V is +5V. What is the reason?
It's likely the equivalent impedance of the three resistors going to the -60V (whose values are not shown)is too high. Eliminate these three resistors. Change the bottom 1kΩ input resistor to 13.7kΩ and the top 1kΩ input resistor to 165kΩ. Connect the top resistor directly to the -60V. That will give you very close to 0V at the resistor junction.........................
I thought about another simple idea (pic attached) where voltages are added at a node. When the voltage goes below -60v, the node is negative. The inverter gives a 5v output signal as a result. When the voltage lies above -60v, the inverter output is 0.
That is the theory. If i simulate it in cadence with dc sweep, the node that should be -5v is +5v. What is the reason?
Hi,
the simple circuit in post3 is correct.
But the resistor values are not.
If you need a 5Mohms input resistor, then (if you want 2.4V output @ -60V input) use a 200k feedback resistor.
Choose an OPAMP that is able to work with common mode input voltage down to 0V (better -0.3V or so).
Klaus
Hi,
Ok, i see. If you like to use the internal Opamp... what is it's common mode input range?
Do you have access to your DAC's Ref voltage?
Using avoltage divider to VCC is possible, but VCC may change.
If it moves 50mV you will see an about 12 fold = 600mV error in your -60V reading.
Therefore it's better to connect the voltage divider to VRef.
For sure if VRef moves this gives error also, But the error is not 12 fold.
******
With knowing the lowes common mode input voltage, you can give this voltage to the +in of the Opamp to work correctely.
All you need is to do a simple subtraction in SW.
Klaus
VICR is : 0 to 3,4V
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