SCB68 how to calculate the correct resistor

[Sara2]

Hi,

sorry I am a new NI user.

I didn't understand how to calculate the correct resistor to apply in scb68 in order to obtain a differential signal.

My application requires an acquisition of a differential voltage channel of 0-5V.

Can I use the ach+ and ach- channels without change anything on scb68?

if no, how can I find correct settings?

I read on scb68 manual page 5-2 "Conditioning Analog Input Channels" but I didn't understand wot to do.

thank you

Sarah

[K C]

Hi Sarah,

It depends a bit an your application.

You can use two resistors minimal 10 times the impedance of your source (or just a 100k or 1M resistor if your source is low impedance) between the analogue input and ground.

These resistors prevent that the input will start to drift.

If your source is high impedance you have to take the resistors into your calculations of the obtained voltage.

Message Edited by K C on 12-12-2007 09:14 AM

[Sara2]

Dear K C,

first thank you very much for your kind reply.

My sensor gives an output impedance of 3 Kohm with power consumption of 180 mW.

The analog signal is a diffrential mode with an aoutput of 0-5 V. Each sensor requires 100mA.

An impedence of 3Kohm is an high impedence?

A resistor of 30 kohm is correct?

The problem is a bit more complex. If I acquire 1 sensor output I am sure of these values.

What could happen if I connect two or more sensors in cascade connections?

The data sheet of my sensors does not explain if the arranged "cascade" connection

is a really serial connection but I believe of yes (not parallel). So, could my total impedence change?

Thank you very much for any help!

Sarah

[Sara2]

Sorry, another question: I choose a cable of 2mt to connect the sensor with the board.

The cable is an RG59 a 75 ohm.

Doaes the cable affect the total impedence of my system?

thank you!

Sarah

[K C]

Hi,

Last question first; the cable impedance is at higher frequencies. So with you (almost) DC application don't worry about this cable impedance.

I would choose resistors of 100k as the lowest value as bias resistors.

You mentioned the power consumption of your sensor (transmitter ?) Does this mean that the sensor is powered through the same lines ?

I don't know how you want to 'cascade' your sensors depends on your sensors. Normally you measure each sensor with a different input.

Can you post (or link to) the datasheet of the sensor ?

Message Edited by K C on 12-12-2007 10:44 AM

[Sara2]

Hi KC, you are very kind to help me 🙂

I choose RG59 (75 ohm) because with these cables I have to transmit 2 digital channel with data at 1 MHz of frquency,

and 1 analog channel and the power lines.

I'm working with hamamatsu sensors (see impedence page 2 of sensor.pdf) to view visible light. I attached here the data sheet.

I have the possibility to connect until four sensor in cascade / serial connection (see page 3 of driver.pdf data sheet) and thus

I can read four analog data train with only one channel.

thank you for any help!

Sarah

[K C]

Hi,

Your RG59 choice is OK. The bias resistors are for the DC part. I forgot to mention this because it is obvious for me.

A quick look at the datasheet tells me that you can cascade the sensor and that one sensor gives the data of all sensors. So this should be no problem.

[Sara2]

Dear KC,

I'm very happy now: finally I (think) understand how to do!

My cable is correct.

I have to apply only one resistor between ACH+ and ACH- of 30Kohm, right?

See the resistor diagram attached ... I have to put my 30kohm resistor to position E

and I have to put two jumpers to position F and G. Finally I have to leave empty other positions?

Thank you for you help!

Sarah

[K C]

Dear Sarah,

Eeeuh.......No. 

You have to use resistors in differential mode to avoid that the input will drift away from the ground level or from each other. This can happen because of leakage of components, connectors, PCB, etc.

Because the impedance of your sensor is rather low this is not likely to happen but to be sure I think it is better to connect both inputs with 1 resistor each to the ground level. I would not use 30k but like I said 100K.

Place the resistors at B and D.

My apology if I was a bit to hasty with the answers and assuming that it was all clear to you 

[Sara2]

Oh, KC, now all is clear to me!

Your help has been very precious to me!

Thank you and ... if I have another question ... I will write again!

🙂

bye

Sarah