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From: Hammy on 29 Nov 2009 08:56
I have a signal between 100kHz to 10MHz sinusoidal DAC output. The
signal starts out at 650mVpp low frequency with a small DC offset of
about 10mV. As the frequency increases the DC offset increases and the
pk-pk voltage decreases. At 10 MHz my signal pk-pk has deteriorated to
about 100mVpp with about 500mV DC offset.

Are there any single chip or small parts count solution AGC that can
maintain a constant 650mVpp output under the above conditions.

I've been goggling AGC and haven't had much luck other then 100 part
count schematics there must be an integrated version?
From: Jan Panteltje on 29 Nov 2009 09:15
On a sunny day (Sun, 29 Nov 2009 08:56:06 -0500) it happened Hammy
<spam(a)spam.com> wrote in <88v4h55c3d0fj1ilbfh6v41mjkqda0r3j2(a)4ax.com>:

>I have a signal between 100kHz to 10MHz sinusoidal DAC output. The
>signal starts out at 650mVpp low frequency with a small DC offset of
>about 10mV. As the frequency increases the DC offset increases and the
>pk-pk voltage decreases. At 10 MHz my signal pk-pk has deteriorated to
>about 100mVpp with about 500mV DC offset.
>
>Are there any single chip or small parts count solution AGC that can
>maintain a constant 650mVpp output under the above conditions.
>
>I've been goggling AGC and haven't had much luck other then 100 part
>count schematics there must be an integrated version?

If the DAC is specified for that frequency, then, if you have control over the digital part,
could be that soft needs improving?
If the DAC is not specified for at least 10MHz get a better DAC.
AGC circuits, the very simple ones with a couple of FETs and or opamp / diodes
and a capacitor to get rid of the DC bias, seem a much more complicated solution,
introduces distortion too.
From: Fred Bartoli on 29 Nov 2009 09:15
Hammy a �crit :
> I have a signal between 100kHz to 10MHz sinusoidal DAC output. The
> signal starts out at 650mVpp low frequency with a small DC offset of
> about 10mV. As the frequency increases the DC offset increases and the
> pk-pk voltage decreases. At 10 MHz my signal pk-pk has deteriorated to
> about 100mVpp with about 500mV DC offset.
>
> Are there any single chip or small parts count solution AGC that can
> maintain a constant 650mVpp output under the above conditions.
>
> I've been goggling AGC and haven't had much luck other then 100 part
> count schematics there must be an integrated version?

Use a better output opamp (yours must be... well...) and adjust the
circuit response so that you're within the specs.
Oh, is your DAC up to the task too?

--
Thanks,
Fred.
From: Fred Bartoli on 29 Nov 2009 09:17
Fred Bartoli a �crit :
> Hammy a �crit :
>> I have a signal between 100kHz to 10MHz sinusoidal DAC output. The
>> signal starts out at 650mVpp low frequency with a small DC offset of
>> about 10mV. As the frequency increases the DC offset increases and the
>> pk-pk voltage decreases. At 10 MHz my signal pk-pk has deteriorated to
>> about 100mVpp with about 500mV DC offset.
>>
>> Are there any single chip or small parts count solution AGC that can
>> maintain a constant 650mVpp output under the above conditions.
>>
>> I've been goggling AGC and haven't had much luck other then 100 part
>> count schematics there must be an integrated version?
>
> Use a better output opamp (yours must be... well...) and adjust the
> circuit response so that you're within the specs.
> Oh, is your DAC up to the task too?
>

Also check your sampling frequency (DAC sinc response).
But the 500mV offset surely indicates your opamp choice is bad.

--
Thanks,
Fred.
From: Hammy on 29 Nov 2009 09:54
On Sun, 29 Nov 2009 15:17:52 +0100, Fred Bartoli <" "> wrote:

>Fred Bartoli a �crit :

>>
>> Use a better output opamp (yours must be... well...) and adjust the
>> circuit response so that you're within the specs.
>> Oh, is your DAC up to the task too?
>>
>
>Also check your sampling frequency (DAC sinc response).
>But the 500mV offset surely indicates your opamp choice is bad.

It's not an op-amp it's the DAC on an AD9834 DDS IC. I want to feed
the signal to an LPF then to an AD8045 for final amplification. I want
to use the AD8045 for variable gain selectable by a POT in the
feedback loop. This is easier done using a consistent input signal. I
think I may have found something AD8330ACPZ.

I'm using the 50MHz part when I use the 75MHz part it probably wont be
as bad.

I cant find any specs in the datasheet about offset vs frequency. It
is possible I damaged the part inadvertently while testing on the
breadbaord;)

Anybody use AD's DDS IC's is this typical?
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