volatage conditioning

[dataq]

I am developing a test bench and have a small problem with signal conditioning.

My signals are DC 60V and i will measure with the NI PXIe-6363 ( +/-10V). I think a galvanic seperation is not nececassary because i measure differentially on a battery.

[Buechsenschuetz]

If you want to avoid any additional circuitry, use a simple voltage divider. The total resistance of both resistors should be around 1/10 of the input resistance of the analog input. However, using the positive half of the input range (0...+10V) only will reduce resolution by one bit.

If you want the full resolution of the ADC you will have to add an offset to the input signal. You will need some op-amp circuitry, a 10V voltage reference plus a power supply for this circuit. Most textbooks on analog design will cover the issue of "summing amplifiers" - if you are a newbie to analog circutry this rather basic circuit might be a good starting point.  

[Buechsenschuetz]

PS. If you use the voltage divider option, when measuring a battery do not forget a bias path fron analog GND to both of the inputs. All analog circuits need a little amount of bias current on their inputs. The manual of the ADC board should cover the issue of properly connecting non-grounded differential voltage sources to the inputs. 

[dataq]

thanks for reply, i think the voltage divider option is the best for me. Do you know where i can buy ready voltage divider for 16+x inputs? I don't want to build something self because i want to duplicate the dynamometer easily with "standard" puchased components .

[Buechsenschuetz]

I did not know they exist, but there ARE some multi-channel voltage dividers available:

 http://www.campbellsci.ca/Catalogue/CVD20.html

It divides the input voltage by 20, so with a 60V input you will have 0...3V output voltage, i.e. you loose another 2 and a half bit resolution.

The same manufacturer has ready-made 10:1 dividers but one channel only.  

[johnsold]

An option which is somewhat intermediate between off the shelf and do it yourself: Buy resistor networks.  You can buy packages (like 16-pin ICs) with 8 identical isolated resistors in each package.  Two packages would give you voltage dividers for 8 channels.  That device has 10 Gohm input impedance (when powered on) so you could use 100 kohm and 10 kohm resistor networks.  Using those values you will get 5.45 V output for 60 V input. These devices cost around $1 (US) in small quantities. I have also seen some networks with voltage divider connections but power dissipation and divider ratio considerations may make these unsuitable.

Lynn