Using the AD844 as an I/V

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tiefbassuebertr

tiefbassuebertr

Member
Joined 2005
tiefbassuebertragung.de
tiefbassuebertr said:
Replacements for AD844/AD846

check out post #5 under
Choosing of best sounding OP AMPs for the lowest possible THD+N -really the best Way?
Click to expand...
the mentioned TI op-amps OPA860/OPA861 in post #19 under
https://www.diyaudio.com/community/threads/i-v-and-analog-output-stage-for-ad1862.299414/
seems to be the closest to the AD844; but to judge it clearly one would need the internal circuit, which is missing as usual with TI - go to the datasheet under
https://www.ti.com/lit/ds/symlink/opa861.pdf
 
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B

BRN

Member
Joined 2006
I have been using the triple AD844 IV with my miro AD1862 DAC and have been very happy with the results. I would like to build one to use the miro PCM63 DAC that I have but wanted to know if I need to change the value of R9 to something like 750 to 1500 ohms?

Also, can I use SparkOS opamps in place of the OPA627s? I have them and just don't want to pay the price to get another pair.
AD844 IV.png
 
abraxalito

abraxalito

Member
Joined 2007
BRN said:
I have been using the triple AD844 IV with my miro AD1862 DAC and have been very happy with the results. I would like to build one to use the miro PCM63 DAC that I have but wanted to know if I need to change the value of R9 to something like 750 to 1500 ohms?
Click to expand...

PCM63 gives twice the output current of AD1862 (+/-2mA against +/-1mA) so yes R9 should reduce to 1k3 (nearest value to half 2k7). However the OPA627 stage has 6dB gain - I would reduce that to 0dB by deleting R5 and reducing R3 to 150ohm. I can't quite see the point of putting in gain to give max 4VRMS output only to reduce it later prior to the poweramp.

I would guess Sparkos opamps would substitute OK for OPA627s but no direct experience of them myself.

P.S. to keep the filtering the same C34 should be increased to 1nF.
 
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H

Hierfi

Member
Joined 2012
In my experiences using the AD844 as an IV, adding a 50 to 100 Ohm resistor between pin 5 and C34 can inhibit oscillations if the power supplies are questionable. I suspect this is caused by the output stage of the AD844 having the middle of the diamond actively driven in order to maximize the slew rate. It seems best if the input source impedance to the output diamond is 50 to 100 Ohms to ground as shorted through C34. It can also help to minimize parasitics at the Tz node from appearing broadband on C34 leading into the buffer.
 
B

BRN

Member
Joined 2006
abraxalito said:
PCM63 gives twice the output current of AD1862 (+/-2mA against +/-1mA) so yes R9 should reduce to 1k3 (nearest value to half 2k7). However the OPA627 stage has 6dB gain - I would reduce that to 0dB by deleting R5 and reducing R3 to 150ohm. I can't quite see the point of putting in gain to give max 4VRMS output only to reduce it later prior to the poweramp.

I would guess Sparkos opamps would substitute OK for OPA627s but no direct experience of them myself.

P.S. to keep the filtering the same C34 should be increased to 1nF.
Click to expand...
Thank you. That is very helpful.
 
T

tonimxp

Member
Joined 2007
abraxalito said:
PCM63 gives twice the output current of AD1862 (+/-2mA against +/-1mA) so yes R9 should reduce to 1k3 (nearest value to half 2k7). However the OPA627 stage has 6dB gain - I would reduce that to 0dB by deleting R5 and reducing R3 to 150ohm. I can't quite see the point of putting in gain to give max 4VRMS output only to reduce it later prior to the poweramp.

I would guess Sparkos opamps would substitute OK for OPA627s but no direct experience of them myself.

P.S. to keep the filtering the same C34 should be increased to 1nF.
Click to expand...
HI Abraxalito what does mening deleting R5, simply pull out to the PCB?
in this case why R33 must be 150R value?
Is possible to use only R13 trimmer for handlyng general offset?
I would be able to avoid also R14 for one only use of trimmers?

Thanks
Antonio
 
abraxalito

abraxalito

Member
Joined 2007
tonimxp said:
what does mening deleting R5, simply pull out to the PCB?
Click to expand...

Yes, precisely. Remove it from the circuit completely.

tonimxp said:
in this case why R33 must be 150R value?
Click to expand...

Just because leaving 1k in circuit would create a lowpass filter with the 1k and the input capacitance of the opamp which reduces its stability. Instead of lowering R33's value you could leave it in circuit and parallel it with a capacitor, say 220pF. That might be more convenient as adding components is normally easier than exchanging them.

tonimxp said:
Is possible to use only R13 trimmer for handlyng general offset?
I would be able to avoid also R14 for one only use of trimmers?
Click to expand...

I think only R13 is needed for offset trimming as the two stages are DC coupled.
 
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