Op amp Dc offset

Presently, i modify a dac which use a CS8417. The output of the CS8417 has output + and output - for each channel. There's 4 electrolytic caps between the dac and and an op amp so there's no Dc coming from the Dac to the inputs of the op amp. At the output of the op amp, there's also two caps.

As many, I hate caps and especially electrolytic caps in the signal. Now, I have two choices to get rid of some caps.

1- I take out the caps between the dac and the op amp so there will be 1.2 VDc at the in+ and in - of the op amp. There will be some DC offset at the output of the op amp so I will need an output cap after the op amp.

2- I keep the caps between the dac and the op amp so there will be 0 VDc at in + and in - of the op amp. There will be 0 VDc at the output of the op amp so the op amp will not need an output cap.

Best should be an op amp which i can adjust the dc offset even if there's Dc at the in + and in - The OPA627 could be a great choice but I don't like the laidback sound of this op amp. I want to use an LM4562 which doesn't have any dc offset adjustement.

Which choice is best 1 or 2 ?

Any other recommendations to adjust the dc offset with op amps which doesn't have any Dc offset control ?

Thanks
 
legarem said:
Presently, i modify a dac which use a CS8417. The output of the CS8417 has output + and output - for each channel. There's 4 electrolytic caps between the dac and and an op amp so there's no Dc coming from the Dac to the inputs of the op amp. At the output of the op amp, there's also two caps.

As many, I hate caps and especially electrolytic caps in the signal. Now, I have two choices to get rid of some caps.

1- I take out the caps between the dac and the op amp so there will be 1.2 VDc at the in+ and in - of the op amp. There will be some DC offset at the output of the op amp so I will need an output cap after the op amp.

2- I keep the caps between the dac and the op amp so there will be 0 VDc at in + and in - of the op amp. There will be 0 VDc at the output of the op amp so the op amp will not need an output cap.

Best should be an op amp which i can adjust the dc offset even if there's Dc at the in + and in - The OPA627 could be a great choice but I don't like the laidback sound of this op amp. I want to use an LM4562 which doesn't have any dc offset adjustement.

Which choice is best 1 or 2 ?

Any other recommendations to adjust the dc offset with op amps which doesn't have any Dc offset control ?

Thanks

Definitely NOT #1. Note that any DC offset at the opamp's input will typically be multiplied by the opamp's gain.

#2 is not quite accurate. Just because the signal path does not provide any DC offset to the input does not mean that there will be no DC offset at the output. There are always bias currents that must flow, into and out of the two opamp input pins. Mismatched impedances seen by the pins can cause the bias currents to create an input DC offset, which is multiplied by the gain. Also, opamps have their own natural input offset voltages, even without the bias currents. Etc. (And note that all of that varies 'by itself' when the temperature of the device changes, especially-badly for FET-input opamps.)

But, you can manually adjust the offset voltage of basically ANY opamp, with a few resistors. See pages 6 and 7 of the following PDF, for simple example schematics showing how to do offset adjustment for all of the common types of opamp amplifier configurations: http://www.national.com/an/AN/AN-31.pdf .

You could (theoretically) also use another opamp to create a 'DC Servo', which would automatically keep the opamp's DC output offset at about zero, even if the input offset is changing, etc. But you would probably want to make sure that the servo's frequency response is down by 100 dB or so above about 10 Hz.
 
nelsonvandal said:
http://headwize.com/ubb/showpost.php?fnum=3&tid=5536&pid=42615&fpage=2

I haven't tried it myself. I'm not clever enough to know if it works just by looking at it.

It might work, but with gain-changing effects that might be significant, depending on the value of the feedback divider resistor. The protection diodes are a nice touch. But the preferred method for a non-inverting configuration is shown on page 6 in the PDF at the link I gave.

In either case, you might also want to add a resistance in series with each power rail, with bypass capacitances to ground between the added resistances and the adjustment pot (equivalent to bypass caps in parallel with the diodes, in the schematic at the headwize link).
 
Tom,

Need to disagree on the offset thing. If the DC at each opamp input is the same (like 1.2V or 2.5V of some DACs) the DC at the output is very, very close to zero. There usually is no gain in these circuits; they are just there to convert the balanced DAC output to single ended, that is what I assume is needed here. Choice # 1 would be the best, no caps, no offset (except a few mV from the opamp itself of course).

As an example, see the data sheet for the balanced voltage output DAC AK4393 for a circuit.

Jan Didden
 
Re: Re: Op amp Dc offset

gootee said:


Definitely NOT #1. Note that any DC offset at the opamp's input will typically be multiplied by the opamp's gain.


This is of course correct, but is it actually a problem?

I like #1 as a simple effective solution, and do it myself. Sonically much better than lytics in the signal path.

As an example, with a single ended voltage out Dac, 2.5 DC volts are at the opamp input, the opamp filter has a gain of 3 (typical) and 7.5 volts DC appear at the opamp output - taken care of nicely by a good 5-10uF polypropylene cap.
If the opamp has supply rails of 12-15 volts then there should be no problems with clipping, and IME you should have no overheating problems.

Although it seems less than ideal to use the opamp as a DC amplifier, are there really any issues with this approach?
 
My point was that with balanced output DACs, the 2.5V or whatever cancels because the opamp is a *difference amp* and there will be NO offset. See pic. You can have your cake and eat it too ;) .

Jan Didden
 

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Re: Re: Re: Op amp Dc offset

float said:



This is of course correct, but is it actually a problem?

I like #1 as a simple effective solution, and do it myself. Sonically much better than lytics in the signal path.

As an example, with a single ended voltage out Dac, 2.5 DC volts are at the opamp input, the opamp filter has a gain of 3 (typical) and 7.5 volts DC appear at the opamp output - taken care of nicely by a good 5-10uF polypropylene cap.
If the opamp has supply rails of 12-15 volts then there should be no problems with clipping, and IME you should have no overheating problems.

Although it seems less than ideal to use the opamp as a DC amplifier, are there really any issues with this approach?


Well, the DC offset does eat into your max undistorted output level. If you have +/-15V supplies and a DC offset of 7.5V, you have on one side only 7.5V left. If the opamp needs a 1V reserve, your max output level is 6.5V peak which is just over 4.5V RMS, instead of some 10VRMS without offset. If you put the cap at the input, you solve this immediately.

Another solution which will allow DC coupling is a servo loop that drives the unused opamp input to whatever necessary to get zero output offset.

Jan Didden
 
Re: Re: Re: Re: Op amp Dc offset

janneman said:



Well, the DC offset does eat into your max undistorted output level. If you have +/-15V supplies and a DC offset of 7.5V, you have on one side only 7.5V left. If the opamp needs a 1V reserve, your max output level is 6.5V peak which is just over 4.5V RMS, instead of some 10VRMS without offset.

I'm with you here, but I'm still unsure if there are disadvantages with this arrangement.
If the output level is around the CD standard 2.0 volt at 0db, there should still be ample headroom. Are there any other factors - distortion due to offset etc that I've missed?
 
Re: Re: Re: Re: Re: Op amp Dc offset

float said:


I'm with you here, but I'm still unsure if there are disadvantages with this arrangement.
If the output level is around the CD standard 2.0 volt at 0db, there should still be ample headroom. Are there any other factors - distortion due to offset etc that I've missed?


I thought you said you needed a gain of 3, that means 3 * 2V RMS which means you get distortion at loud passages - but you can get the gain down to 2 or so, of course. I am not aware of any poweramp that needs more than 1 or 2 VRMS for full output power, anyway. You could even go with unity gain.


An other issue is whether the opamp has increased distortion when it is working with offset. Obviously the signal will be far away from the nominal middle between the supplies. But I don't have any experience with that - it may be a non-issue.

Jan Didden
 
It's quite normal to have headroom of 10db between maximum signal level and clipping in the active stage. Some strive for close to 18db of headroom.
For a CD source with maximum signal in the range 2Vac to 2.3Vac the peak voltage is about 3.2Vpk.

Allowing 10db of headroom requires about 10Vpk to just reach clipping.

If 2.5V of offset is presented to the input, then an opamp operating off a normal +-15Vdc supply will have compromised headroom.
 
janneman said:
Tom,

Need to disagree on the offset thing. If the DC at each opamp input is the same (like 1.2V or 2.5V of some DACs) the DC at the output is very, very close to zero. There usually is no gain in these circuits; they are just there to convert the balanced DAC output to single ended, that is what I assume is needed here. Choice # 1 would be the best, no caps, no offset (except a few mV from the opamp itself of course).

As an example, see the data sheet for the balanced voltage output DAC AK4393 for a circuit.

Jan Didden

Thanks, Jan. That'll teach me to not comment when I have no idea what the circuit is. :)