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jhenderson
Site Admin
Joined: 07 Mar 2006
Posts: 1177
Location: So. Cal. USA
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 Posted: Mon May 04, 2009 7:55 am Post subject: DC-coupled outputs? |
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| Quote: | Is it possible to configure the outputs of the PMC-TX module for DC-coupled operation? If not, is it possible to extend the low-frequency response of the system?
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Because the output amplifiers on the PMC-TX D/A channels are powered from a single 5V supply, they cannot be DC coupled without causing a large offset voltage, approximately 2.5V unloaded, or 1.25V into a 50 ohm load (i.e. into a 50 ohm load, the signal would swing from 0.75V to 1.75V).
If this offset is acceptable in your application, I can provide a modification to remove the AC-coupling caps from the output amplifiers. Alternatively, the current low frequency cutoff sits at about 23kHz, -3dB. We might be able to reduce that frequency: what minimum frequency do you need to pass in your application?
| Quote: | We need to pass a baseband composite video signal. It is assumed there is a DC restore at the destination. In the past we have used an output stage that is an OP-AMP with a +5V supply and GND (single supply). This drives a 75 Ohm series source impedence then a 2200 uF Cap. The load is 75 Ohms.
+5V + -
OP-AMP -> -> 75 Ohm ->-> 2200uH ->-> cable ->-> 75 ohm load
GND
The Op-Amp has a gain of 2
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We can't mechanically fit a 2200uF cap on the existing board design. The existing output caps are 0402 sized, we might be able to shoehorn an 0603 device in the same space, but that would only give us a few tens of uF at most.
I suspect the best solution would be to DC-couple the onboard amp, and build a cable with the 2200uF series cap included to get the low frequency highpass filter. I glanced at Pasternack's website and they don't appear to have a DC block coupler that goes lower than 10 MHz. MiniCircuits goes down to 100kHz.
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Jim |
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shackleton
Joined: 20 Oct 2009
Posts: 10
Location: Italy
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| Posted: Wed Nov 18, 2009 2:13 am Post subject: Tx boards with DC output |
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Dear Sirs,
my customer has purchased four TX boards with DC output.
However, running Testbed, the output waveforms are all centered around zero voltage, instead swinging +/-1V around 2.5V, so my conclusion is the boards are shipped without the DC modification.
My questions is: if the modification isn't too complex -I suppose is not- is possible to have the instructions to apply the change on our labs? Otherwise we should return you the boards to do this.
Thank you in advance. |
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jherring
Joined: 12 Apr 2006
Posts: 36
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| Posted: Thu Nov 19, 2009 9:04 am Post subject: |
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We don't offer a DC-coupled version of the PMC-TX module, so I'm not sure what hardware your customer ordered. Nevertheless, it sounds like they have a standard board.
The component changes would be as follows:
1) Remove C83, C84, C85, C88, C91, C102, C103, C109, C112, C113, C126, C127, C128, C129, C132, C135, C136, C137, C151, and C155.
2) Install shorts at the above locations.
3) Change U19, U20, U22, and U23 to THS4508RGTT devices. |
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shackleton
Joined: 20 Oct 2009
Posts: 10
Location: Italy
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| Posted: Fri Nov 20, 2009 6:32 am Post subject: |
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Hi jerring,
my customer ordered the boards following the information reported on this forum ("The output in the DC coupled case would swing +/-1V around 2.5V... If the behaviour is acceptable, Innovative can modify the module to operate in this manner").
I suppose something was going wrong with Innovative distributor.
Anyway we are able to modify the boards but we need the components mounting plan and -more desiderable- the schematic about this stage. In alternative please suggest me a procedure to return you the boards for reworking.
For your information, our application needs to generate four analog signals 0V centered and DC coupled; so we are going to implement an adapter board that, in addition to some digital circuits, makes the offset corrections reporting the null signal to 0V.
Thank you again in advance. |
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jherring
Joined: 12 Apr 2006
Posts: 36
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| Posted: Fri Nov 20, 2009 9:14 am Post subject: |
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If you want to have us do the modification, please contact Peg Todd at our office at ptodd@innovative-dsp.com.
The assembly drawings are attached. |
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shackleton
Joined: 20 Oct 2009
Posts: 10
Location: Italy
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| Posted: Fri Jul 30, 2010 5:13 am Post subject: |
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Hi jherring,
we are followed your instructions to modify a TX board but, we found some -not identified- capacitors soldered between the following couples of R
and C:
C83 - R68,
C85 - R78,
R92 - C103,
R84 - C102,
R99 - C126,
R109 - C128,
R115 - C135,
R123 - C137.
Our question is: the added capacitors need to be maintained or need to be removed? We have attached a image where we highlight the new added capacitors.
Thank you very much in advance for your suggestions.
| jherring wrote: | We don't offer a DC-coupled version of the PMC-TX module, so I'm not sure what hardware your customer ordered. Nevertheless, it sounds like they have a standard board.
The component changes would be as follows:
1) Remove C83, C84, C85, C88, C91, C102, C103, C109, C112, C113, C126, C127, C128, C129, C132, C135, C136, C137, C151, and C155.
2) Install shorts at the above locations.
3) Change U19, U20, U22, and U23 to THS4508RGTT devices. |
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jherring
Joined: 12 Apr 2006
Posts: 36
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| Posted: Fri Jul 30, 2010 7:50 am Post subject: |
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| The additional capacitors should be left in place. |
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