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From: Hammy on 29 Nov 2009 10:03
On Sun, 29 Nov 2009 09:54:09 -0500, Hammy <spam(a)spam.com> wrote:

>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?

It seems that this is normal for these parts. This guy is using an
opamp (IC6 OPA843) to adjust the output of the DDS before the LPF.

SEE

http://i46.tinypic.com/65v4ut.png

I want to be able to use an AGC Wideband Op-Amp there instead.
From: Jan Panteltje on 29 Nov 2009 10:24
On Nov 29, 4:03 pm, Hammy <s...(a)spam.com> wrote:
> On Sun, 29 Nov 2009 09:54:09 -0500, Hammy <s...(a)spam.com> wrote:
> >On Sun, 29 Nov 2009 15:17:52 +0100, Fred Bartoli <" "> wrote:
>
> >>Fred Bartoli a écrit :
>
> >>> 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?
>
> It seems that this is normal for these parts. This guy is using an
> opamp (IC6 OPA843) to adjust the output of the DDS before the LPF.
>
> SEE
>
> http://i46.tinypic.com/65v4ut.png
>
>  I want to be able to use an AGC Wideband Op-Amp there instead.

How accurate does it have to be?
You can use AC coupling to get rid of the offset,
and a 1 of 8 analog multiplexer with some resistor divider to split
the range into 8 gain stages.
Control the mux (74HC5051 ?) from the same micro that you use to
control the DDS.
2 muxes ->16 ranges.... snall part count.

From: Joerg on 29 Nov 2009 12:40
Hammy wrote:
> 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?


Yes, but it's expensive:

http://www.analog.com/static/imported-files/Data_Sheets/AD8367.pdf

If on a budget you could build one around a uA733 or similar chip, add a
simple detector and a FET as the gain control element. If your budget is
super-tight use a dual-gate MOSFET which allows the gain control range
you are looking for. One gate gets the signal, the other the control
voltage.

Here is another trick to save a buck: Characterize the signal drop
versus frequency. Then use a simple gain control stage and place a 2nd
DAC. This 2nd DAC is used to set the gain control input and is updated
whenever you update the frequency.

--
Regards, Joerg

http://www.analogconsultants.com/

"gmail" domain blocked because of excessive spam.
Use another domain or send PM.
From: Jim Thompson on 29 Nov 2009 13:10
On Sun, 29 Nov 2009 09:40:02 -0800, Joerg <invalid(a)invalid.invalid>
wrote:

>Hammy wrote:
>> 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?
>
>
>Yes, but it's expensive:
>
>http://www.analog.com/static/imported-files/Data_Sheets/AD8367.pdf
>
>If on a budget you could build one around a uA733 or similar chip, add a
>simple detector and a FET as the gain control element. If your budget is
>super-tight use a dual-gate MOSFET which allows the gain control range
>you are looking for. One gate gets the signal, the other the control
>voltage.
>
>Here is another trick to save a buck: Characterize the signal drop
>versus frequency. Then use a simple gain control stage and place a 2nd
>DAC. This 2nd DAC is used to set the gain control input and is updated
>whenever you update the frequency.

Years ago, I used a temperature-driven look-up table, controlling a
varicap, to make crystal oscillators as stable as oven-controlled...
without the power requirements of an oven.

...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
From: Vladimir Vassilevsky on 29 Nov 2009 13:39


Jim Thompson wrote:


> Years ago, I used a temperature-driven look-up table, controlling a
> varicap, to make crystal oscillators as stable as oven-controlled...
> without the power requirements of an oven.

BTDT. The problem is there are many more of unstable parts in the
control loop, then in case of simple OCXO. Even if the compensation is
initially perfect, it will degrade with aging.


Vladimir Vassilevsky
DSP and Mixed Signal Design Consultant
http://www.abvolt.com
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