[SOLVED] reading negative DC voltage from converter with ADC

[P.Copper]

Hi y'all,

I have a converter that is designed to output a -100V but now I find it challenging to read that on ADC. My first approach was to use an inverting amplifier with a gain of 1, the problem is that i'm not sure if the op-amp will be able to handle 100V because most of the amplifiers are rated at low voltages. Can anyone help me on this. Thanks

 

[FvM]

You'll use a circuit that doesn't connect the -100V directly to an amplifier pin but drops the voltage across a resistor. A basic inverting OP amplifier will already achieve this. https://en.wikipedia.org/wiki/Inverting_amplifier#Inverting_amplifier

 

[KlausST]

Hi,

There a countless inverters, countless ADCs, countless OPAMPs and countless different circuits.

Without part numbers, schematic with values it is hard to give you good assistance.


Klaus

 

[P.Copper]

You'll use a circuit that doesn't connect the -100V directly to an amplifier pin but drops the voltage across a resistor. A basic inverting OP amplifier will already achieve this. https://en.wikipedia.org/wiki/Inverting_amplifier#Inverting_amplifier

So you're saying that "let's say I use a voltage divider as a sensor to output -5V from input of -100V then I use an inverting amplifier to get a positive 5V" ?

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Hi,

There a countless inverters, countless ADCs, countless OPAMPs and countless different circuits.

Without part numbers, schematic with values it is hard to give you good assistance.


Klaus

I'm using ADC of dsPIC30f4011, and LM358N op-amp.

- - - Updated - - -

Hi,

There a countless inverters, countless ADCs, countless OPAMPs and countless different circuits.

Without part numbers, schematic with values it is hard to give you good assistance.


Klaus

I'm using ADC of dsPIC30f4011, and LM358N op-amp.

 

[schmitt trigger]

In addition to what other posters have mentioned, the opamp MUST be supplied by both positive and negative supplies.

If you only have, let's say, +5v available, use a ICL7660 voltage converter to obtain the -5v.

 

[Audioguru]

The PNP transistor inputs of an LM358 dual opamp work perfectly when they are at the voltage of its negative supply pin so it can be a single positive supply inverter when it has a negative voltage to its input resistor. Its output goes as high as +5V only when its supply is about +6.5V.

 

[P.Copper]

If you only have, let's say, +5v available, use a ICL7660 voltage converter to obtain the -5v.

Is it still applicable to get +5V from -5V using the ICL7660? I'm asking this because in the datasheet they only mention converting positive voltages to negative voltages. I'm interested in converting from negative to positive voltages.

 

[BradtheRad]

Here is an easy method to lift a negative signal into the +5V range.

Mistakes are easy with this method. For instance never disconnect the +5V, because that will cause the output to go to -100V.

**broken link removed**

The 1M resistor is unnecessary. It's purpose is to illustrate how the output is ground referenced.

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I forgot to add... your input signal must be able to sink a few mA. The 33k resistor dissipates a few hundred mW of heat.