wrong voltage while switching resistive load through ni-pxi 2520

[aroonkriss]

Hi all,

i have been monitoring DC voltage value of a pulsed waveform. The load resistor is 1k. This load resistor need to be switched. For this switching i am using ni-pxi-2520 and an agilent DMM (GPIB) is switched across this load using ni-pxi-2575. But there is an anomaly in the measured voltage. If i measure, the same voltage without any switching(both load and DMM), i will get the voltage within specification. but if i use any of the switch, the voltage changes. I am attaching the connection details and part of the circuit in which i need to measure the voltage and different voltage in various conditions. Any help would be appreciated.

Thanks in advance

Arun

[aroonkriss]

Hi all,

is it an impedance matching problem since i am measuring the voltage between two points differentially? 

regards 

arun

[aroonkriss]

Hi all,

i monitored the pwm in both cases through a digital oscilloscope. if i connect the resistance without realy, the mean is -1.42V (positive width 174 micro sec, -ve width 276 micro sec, period 450 micro sec, max 25.8V and min -25 V). but with the same input settings, the mean changes to -2.48V (positve width 172 micro sec, neg width 278 micro sec, period 450 micro sec, max 25.8V, min -25V).  i could not figure out the problem. if anyone can help me please respond. any help would be appreciated

Thank you

Arun

[aroonkriss]

hi all,

i have removed the load resistor relay switching. Now i have a load resistor connected directly across the output points. and one ni-pxi2575 relay (95X1 mode ) is used to connect the Digital Multimeter  across the output points. Still having the problem. Do the relay have any effect on the pwm signal. anybody please respond. i am really in trouble

Thank you

Arun