question about the voltage follower of floating signal differtial measurement system

[datoudream]

Hi.

I am trying to acquire 3 signals from 3 photodiodes and their preamplifiers through PCI-6281. I think it should be a floating signal( because neither the circut nor the power supply is linked to the earth), and use differential measurement system. According to Figure 5 of the paper "Using a Unity Gain Buffer (Voltage Follower) with a DAQ Device", I shall connect the 2 terminals of the voltage signal to 2 voltage follower.

But the negative terminal is power ground, and I think it's unusual to make ground as amplifier input. Shall I do as figure 5 connecting ground to voltage follower?

Also I want to ask about the power supply in figure 5. What's the COMMON terminal? Is it ground? And how is the 5V terminal? Which terminal of the DAQ card is connected to 5V?

Attachment is a diagram of my system. And I want to know whether I can connect the ground of photodiode circut and that of measurement system.

I am just confused by so many kinds of ground.

Thanks for any help.

帖子被datoudream在09-05-2007 12:31 AM时编辑过了

[Henrik_Volkers]

Hi,

Since you already have an amplifier, (your photo-current to voltage amp) I would try it without these additional voltagefollower, and due to the I2U amp you choose, the signal is already ground referenced. 

Your I2U amp should be close to the sensor, so your SEPERATE groundline ( I like to use shielded twistet pair*) together with the output can go into the differtial input of your DAQ. Add a 1M from DAQ Measurement GRND to your - Input . Where to connect the shield is a question of taste, most of the time it's recommended to connect only one side to common ground.

And Yes, in simple theorie you already have a ground referenced signal, that needs only a single ground referenced  DAQ input. BUT since in pratice your (resistive) groundline is not free of currents you will get noise on it, that can be greatly reduced by measuring in differential mode.

*)I would go with a CAT(7) cable , cheap, easy awailable and it comes with 4 pairs of seperately shielded twisted pairs.   

[datoudream]

Thanks to Henrik. I will use shielded twisted pairs.

But I cannot understand something.

First, why the signal is ground referenced? The power supply of the amplifier

is not connected to earth or building. The GND is just reference ground. So I

think it floating signal. Do u mean the DC power supply is connected to the

earth? And any circut including amplfier is grounded signal? And what signal

is floating signal?

Second, voltage follower is necessary for the out impendence of the I2V

converter is about 1MW.

Third, I think it's wrong to connect a ground referenced signal to a single

ground referenced  DAQ input as u say.

So, I want u to explain about the ground. What's earth or building ground and

reference ground? Is just battery powered circut floating signal? Does DC

power supply usually connect to the earth or building?

Thanks.

[Henrik_Volkers]

"First, why the signal is ground referenced? The power supply of the amplifier

is not connected to earth or building. The GND is just reference ground. So I

think it floating signal. Do u mean the DC power supply is connected to the

earth? And any circut including amplfier is grounded signal? And what signal

is floating signal?"

The output current of your photodiode is floating.  After your I2U Amp it's referenced to GND (and has a quite low impedance).  

You say GND is floating, well it's tied to AIGND with R_B .

"Second, voltage follower is necessary for the out impendence of the I2V

converter is about 1MW." 

??? ONE MEGA WATT ???

"Third, I think it's wrong to connect a ground referenced signal to a single

ground referenced  DAQ input as u say." 

Read my answer again. It's not wrong (hey, what's wrong anyhow) sometimes it's the only way you can do it (if you don't have the diff option or not enough channels left ) . It's better practice to read it in diff mode.

"So, I want u to explain about the ground. What's earth or building ground and

reference ground? Is just battery powered circut floating signal? Does DC

power supply usually connect to the earth or building?   "

If it comes hard, GROUND is the the point YOU define as. In the measurement labs at university  we had a nice homebrew REFERENCE GROUND build with a good copper clamp an a  flowerpot... and it was important to keep it wet!!