Purpose of Parallel Capacitor with AD811 I/V circuit? - diyAudio
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Old 9th February 2005, 12:25 AM   #1
diyAudio Member
Join Date: Jul 2003
Location: Indiana
Default Purpose of Parallel Capacitor with AD811 I/V circuit?

Hello to All:

I have a DAC that uses the AD811 chip for the I/V
conversion. It has a 5.23 K ohm resistor running
from the output to the inverting input to generate
the voltage, and a 47 pF capacitor in PARALLEL with
the resistor.

Can someone tell me what the purpose of the cap
is in the circuit? Does it help to form some sort of
first order low pass function (1/(2 pi X RC))? Or
is it there to perform some sort of feedback comp.
or stability for higher frequencies?

The overall DAC output circuit also has the typical
configuration of several first order low pass poles
between the AD811 and an AD817 output op-amp.

Am asking this because (if the cap forms a low pass
function with the resistor) I want to change the
value to 330pF to create a lower cut off freq., and
make the overall circuit just a simple first order
low pass filter. But also do not want to change
the value if it is important to the stability of the
AD811 I/V chip.

Many Thanks in Advance!

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Old 9th February 2005, 09:15 AM   #2
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Kochkurov Maxim's Avatar
Join Date: Dec 2003
Location: Russia

I recommend to take off this capacitors from your DAC. This capacitors may be a cause of AD811 oscillation. Current feedback amplifier, as AD811, AD8009, etc., don't like capacitors in feedback circuits and very often oscillate and burn off, if capacitor more than 1-2 pF is installed.
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Old 9th February 2005, 11:43 AM   #3
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Ouroboros's Avatar
Join Date: Dec 2003
Location: Nottingham UK
If the circuit used a voltage-feedback op-amp, the the capacitor would simply add a first-order high-frequency roll-off.

BUT...as the previous poster says. Current-feedback amps,like the AD811, musn't have a capacitor connected in this way. It may make the amplifier unstable.
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Old 9th February 2005, 05:09 PM   #4
banana is offline banana  Hong Kong
diyAudio Member
Join Date: Feb 2003
Location: Hong Kong
5.23K I/V resistor, AD811 I/V and AD817 LPF.... that sounds like an Assemblage DAC2.X

the 47p cap is a standard lead compensation approach for voltage feedback opamp. But for current feedback opamp like AD811, the 47p shouldn't be there.
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