Hi,
Noob question, I'm afraid. Could someone give me some pointers on how I might scale/offset 0-5v output from an DAC into the range -5 to +5v?
Cheers,
a|x
[0-to-5V] into [X2 amp] into [summing amp]
[u]Here[/u] is a schematic for a summing amplifier. Since you actually want to subtract, you need to sum a negative voltage with the positive signal bias.
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Since you want to swing between +5V and -5V the power supply for your op-amp needs to be greater than that. (I'd power it from +/-12 or +/-15V.)
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Summing amplifiers are inverting amplifiers. If that's a problem in your application, follow the summing amp with an inverting buffer (a 2nd op-amp with a gain of -1). Or, you can invert the digital data before the DAC.
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Since you are summing only two inputs you only need two input resistors.
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To get a signal gain of 2 (technically -2), the signal-input resistor value should be half the feedback resistor value.
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Calculate the bias-input resistor according to your negative supply voltage, and connect it to your negative supply. For example, to subtract-out the 5V bias (it's 2.5V before being amplified x2) with a-12V power supply, you need a gain of -5/12 (-0.417). That would mean an input resistor 12/5 (2.4) times the feedback resistor.
If DC preservation is not important (e.g. audio), you could use an inverting opamp with a 2x gain and an input capacitor.
Followed by another inverting opamp if you want to preserve the phase.
Leo..
Hi,
What is the DAC part number?
What needs the -5V to +5V range input?
Tom..... 
Educated guess: MCP4822
Hi,
thanks very much for all your replies, guys. Plenty to get my teeth into there, I think.
@TomGeorge/Wawa yes, my plan was to go with two MCP4822 dual 12-bit DAC. The +-5v output range is because the circuit is intended to form part of a Eurorack modular synth module. The Eurorack standard states that modulation signals such as those I intend my module to produce should be in the -5 to +5v range, so they can be used to modulate parameters of other modules in both positive and negative directions.
@DVDdoug thanks very much for the link. I will study that, and see if I can get my head around it. I can easily invert the DAC output in software, so the extra inverting buffer shouldn't be required.
Thanks again, guys!
a|x
Ah, the MCP4822 appears to have a maximum output voltage of 4.096v. I guess it's still possible to do what I want, though, with some adjustment to the resistor values. I can use the +/-12v power from my Euro PSU, and step that down to 5v to power the ATMega328.
Would this quad op-amp do the trick?
@DVDdoug so…
I'd first need to sum my original 0 > 4.096v DAC output with -2.048v, before boosting the result through the opamp.
Can I use the same -12v from the Euro power-supply I'm using to power the opamp to derive the bias voltage?
a|x
toneburst:
Would this quad op-amp do the trick?
You only need a single op amp. Make an inverting amplifier with a gain of 2 and connect the + input to + 2.5 V using a potential divider of two equal resistors off the 5V supply.
Russell.
Ah, so instead of connecting the + of the opamp to ground, connect it to +2.5V?
toneburst:
Ah, so instead of connecting the + of the opamp to ground, connect it to +2.5V?
Yes. Of course you still need a negative rail supply.
Russell.
@russellz Like this?
'2xr' is a mistake- I was assuming the input is in the range 0 - 5V when I wrote that, but of course it isn't...
Could you suggest values for the two resistors that would work? I understand that the ratio between the two resistors determines the opamp gain, but do the actual values matter?
a|x
toneburst:
Could you suggest values for the two resistors that would work? I understand that the ratio between the two resistors determines the opamp gain, but do the actual values matter?
The resistor values are not critical. You could use 11 k and 27 k to give you close to 5 V. I would go for +- 9 V or +- 12 V for the supply rails though.
Russell.
@russellz thanks! That looks easy enough..
a|x