(Updated 26/10 2012)


Dear All

I have solved the power problem and here comes my design purpose, schematic and some questions:

I would like to control an optical modulator by a DAC. The DAC needs a precision of 0.1mV. The DAC will be connected with the modulator through a coaxial cable.

1.One end of modulator is connected with ground. In order to avoid ground loop, I think differential output(from LP filter after DAC) should be connected with the modulator, is that correct?

2. After DAC, there will be a low-pass filter, before connecting with the optical modulator. The output of the low pass filter will be differential pairs. Which kind of DAC should be used to connect with the differential output LP filter, a differential DAC or a voltage output DAC?

3. I have read in a tutorial (http://www.hit.bme.hu/~papay/edu/Acrobat/GndADCs.pdf) that the connection between DAC with data bus should be buffered to reduce digital return current. Is it also true for a microcontroller based application? If so, what kind of buffers should be used?

My current design: MicroController (No Buffer)---> Single end output voltage DAC--->single input differential output low pass filter---->Coaxial cable---->Optical modulator.

It would be really appreciated if you could give me any suggestions/advice on the questions/design.
BTW, I have watched 5 things you need to know about DACs. It is very useful.

Thank you very very much.

Regards
Richie

 

Careful consideration to what currents go where is better. Given a clean DAC signal, you should be able to make a "star" ground connection between a single ended DAC output and the modulator.

 

Just updated the thread.

Really appreciate if you could have a look.

 

We have no idea what your ground network looks like. you need to provide a picture and/or a detailed description

 

Just updated the thread.

Really appreciate if you could have a look.

You state you want to build an digital to analog converter (D2A).

The doc you reference is for an analog to digital converter (A2D).

One of these things is not like the other...

A D2A does not need to drive a bus as it has no output logic lines.

 

You state you want to build an digital to analog converter (D2A).

The doc you reference is for an analog to digital converter (A2D).

One of these things is not like the other...

A D2A does not need to drive a bus as it has no output logic lines.

If you read the doc he referenced, you'll see that it addresses both ADCs and DACs.

 

Richiechen, is your optical modulator a commercial product? If so, can you link to the datasheet?

 

Richiechen, is your optical modulator a commercial product? If so, can you link to the datasheet?

Ron H

Thanks for your reply.
Here comes the datasheet of the optical modulator:
http://www.jdsu.com/ProductLiterature/ddmz10g_ds_cc_ae.pdf

In page 3, there is a schematic of the modulator. The DC bias pin is the voltage input pin which receives signal from DAC.
The DC bias pin should be connected with coaxial cable and the outer layer of the DC bias pin is connected with modulator case(grounded). Therefore, one end of modulator, which is the signal receiver, is already connected with ground.

BTW, I dont quite understand the inside structure of the modulator. Could you please give me a hint?


Regards
Richie

 

You state you want to build an digital to analog converter (D2A).

The doc you reference is for an analog to digital converter (A2D).

One of these things is not like the other...

A D2A does not need to drive a bus as it has no output logic lines.

Ernie

You are right saying that ADC needs buffer because of the driving capability.

However, as the document says, DAC needs a buffer to reduce the digital return current from ground. Without a buffer, the current has to be returned to MCU. With a buffer, the current could be returned to the buffer directly and the current will be smaller.

My question is, in a microcontroller bases application, is the buffer necessary?　And where to find the buffers?

Thank you.

Regards
Richie

 

If you read the doc he referenced, you'll see that it addresses both ADCs and DACs.

Sorry, after scanning several pages of inapplicable information I stopped looking.

 

Richie, do you know that the modulator is optimized for RF application? It appears to have bandwidth down to DC, but It says nothing about linearity. Google "mach-zehnder modulator tutorial" and click on the second link on the page. It says that the transfer function is nonlinear, compressing the positive and negative peaks (see the section on Mach-Zehnder modulators).

And where will you get an optical demodulator with .01mV precision (whatever that means)?
What are you planning to do with the modulated signal?

 

Richie, do you know that the modulator is optimized for RF application? It appears to have bandwidth down to DC, but It says nothing about linearity. Google "mach-zehnder modulator tutorial" and click on the second link on the page. It says that the transfer function is nonlinear, compressing the positive and negative peaks (see the section on Mach-Zehnder modulators).

And where will you get an optical demodulator with .01mV precision (whatever that means)?
What are you planning to do with the modulated signal?

You are right saying that the modulator is non-linear. Actually I am trying to use the non-linearity to control the modulator. I will send a test signal to the modulator and analyze the distortion of the response.

The modulator needs a DC bias voltage to operate correctly. And the bias voltage will need a step size of 0.1mV.

If this is the case, do you think differential output is necessary?


Regards
Richie

 

OK, now I get it.
The signals that need to be differential are the RF inputs. The grounding requirements for the bias depend in part on how far away the modulator is from the bias source.
As I said in post #4,

We have no idea what your ground network looks like. you need to provide a picture and/or a detailed description.

 

OK, now I get it.
The signals that need to be differential are the RF inputs. The grounding requirements for the bias depend in part on how far away the modulator is from the bias source.
As I said in post #4,

The distance depends on the customer's need. Can I just make the output differential?

If so, a single end DAC is better + differential LP filter is better
OR
a differential DAC + differential LP filter is better?

Thank you

 

The distance depends on the customer's need. Can I just make the output differential?

If so, a single end DAC is better + differential LP filter is better
OR
a differential DAC + differential LP filter is better?

Thank you

Are you thinking of connecting one side of the differential DC bias to channel 1, and the other to channel 2? Would you make the differential voltage symmetrical around ground, or what? I think you need to contact JDSU and get some detailed applications information. You don't know enough about what you are trying to do to make a commercial product.

 

Are you thinking of connecting one side of the differential DC bias to channel 1, and the other to channel 2? Would you make the differential voltage symmetrical around ground, or what? I think you need to contact JDSU and get some detailed applications information. You don't know enough about what you are trying to do to make a commercial product.

You are right!! I should contact JDSU!

What I planned to do is to make the DAC output differential. It means DAC output has no end connected with ground. Both end will be connected with modulator bias 1.
It means one end of DAC will be pulled to ground by the modulator. However, DAC itself has no end connected with ground. Therefore, no ground loops will be formed.

Richie

 

You are right!! I should contact JDSU!

What I planned to do is to make the DAC output differential. It means DAC output has no end connected with ground. Both end will be connected with modulator bias 1.
It means one end of DAC will be pulled to ground by the modulator. However, DAC itself has no end connected with ground. Therefore, no ground loops will be formed.

Richie

Are the modulation signals coming from the same board as the DAC? Their shields need to be grounded.

 

Are the modulation signals coming from the same board as the DAC? Their shields need to be grounded.

The modulation signals are not from DAC. DAC only has DC bias signal.

Why modulation signal's shield needs to be grounded?

Richie

 

The modulation signals are not from DAC. DAC only has DC bias signal.

Why modulation signal's shield needs to be grounded?

Richie

If it's not grounded, there is no return current path for the signal, which is on the center conductor. They could be grounded through a capacitor, though.

 

If it's not grounded, there is no return current path for the signal, which is on the center conductor. They could be grounded through a capacitor, though.

The modulation signals are sent through coaxial cables. The shield of the coaxial cable connects the modulator ground and the V- of the modulation signal.

Excuse, I still confuse why the signal has to be grounded.

Thank you.

Richie