Integrated AGC 100kHz-10MHz

[Hammy]

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?

 

[Fred Bartoli]

Hammy a écrit :

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?

 

[Fred Bartoli]

Fred Bartoli a écrit :

Hammy 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.

 

[Hammy]

Fred Bartoli a écrit :

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?

 

[Hammy]

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.

 

[Joerg]

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.

 

[Jamie]

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 sounds like you have a cos/sin of your output shifting amplitude.
This would thus be the same as changing the duty cycle and causing
the offset you're seeing.
I wonder if you have a unity stage for the DAC to prevent excess
loading, do to load capacitance ?

This all maybe a shot in the dark but it's my shot

 

[john jardine]

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?

With a 50MHz clock the signal would normally drop to maybe 500mV at 10MHz
but that's assuming you're measuring using a 10:1 scope probe with say 13pF
tip capacitance.
The 'FSadjust' and 'Rset' can be used for AGC. Put a DC 'level set' voltage
on the Rser resistor end that normally connects to 0V. (OV=max out). Idea is
to throttle the output swing to (say) 1/2 of normal, which then allows a
+50% increase to compensate for losses.

 

[Hammy]

Yes, but it's expensive:

What isn't

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

I was thinking of putting a digital pot in the AD8045 feedback loop.
But by the time all is said and done 6 Bucks for the AD8637 looks
pretty good.Newark sent me out a 25% off flyer so it's a good time to
buy parts.

I'm using an AD9834 DDS IC to build my own signal generator controlled
from the serial port. The parts pretty impressive for $7 sine, square
,triangle output can perform various modulation with minimal parts
,ability to sweep frequency range defined step size and rate and a
75MHz clock. I know AD make much faster ones but a 10MHz output max is
good for me.

Trying to keep it in the $35 to $50 range. I was going to buy a cheap
$150 - $250 Chinese one but decided to build my own.

 

[Hammy]

With a 50MHz clock the signal would normally drop to maybe 500mV at 10MHz
but that's assuming you're measuring using a 10:1 scope probe with say 13pF
tip capacitance.
The 'FSadjust' and 'Rset' can be used for AGC. Put a DC 'level set' voltage
on the Rser resistor end that normally connects to 0V. (OV=max out). Idea is
to throttle the output swing to (say) 1/2 of normal, which then allows a
+50% increase to compensate for losses.

Thanks John.

I'll try playing around with that.

 

[Joerg]

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.

You did WHAT? A LUT? Meaning digital stuff? <gasp>

Now go, stand in the corner, and be ashamed. Tsk, tsk, tsk ...

 

[Joerg]

The client, Saunders and Associates (builders of crystal testing
equipment) did the LUT. I did the analog and A/D/A interfaces.

Good. Your are redeemed. Welcome back to the analog club

 

[JosephKK]

Thanks John.

I'll try playing around with that.

With all that has been said, i suspect a miswire or some circuit
problem. Both the DSS and the first OPA should be up to the task
(without the stated aberrations). Though it could very well be a
layout issue as well. Since the DSS is current output are you using
an appropriate termination and coupling to the OPA?

 

[Hammy]

With all that has been said, i suspect a miswire or some circuit
problem. Both the DSS and the first OPA should be up to the task
(without the stated aberrations). Though it could very well be a
layout issue as well. Since the DSS is current output are you using
an appropriate termination and coupling to the OPA?

Actually I'm embarrassed to say I had my probe on 1x (I know stupid).
I really have to stop doing that it's getting ridiculous.

I'm just working on the power supply now.

On the plus side I learnt a lot about AGC ccts and what's out there
for future reference.

Thanks to all who helped.