| Electronics > Projects, Designs, and Technical Stuff |
| What the ???!!! 920MHz Xbee interferes with op amp circuit |
| (1/1) |
| Nite_Shift:
Wondering if anyone has any thoughts on this.. I have a simple non inverting amplifier circuit (LM7321, with a gain of 4.3 (R1=33K, R2=10 K)) running on my lab power supply. The non inverting input is fed with a voltage divider off the rails. When my battery powered 920MHz xbee transmits at 100mm distance, the op amp output dips up to 1V for the amount of time the xbee is transmitting. If the xbee is 1m away, the output dips by about 100mV. I have tried a couple of other op amps, same result!\ Just to be clear, The rails and the non inverting input are clean, and the dip in voltage lasts the length of the transmission and is not noisy. Very strange. Attached screen shot of op amp output. transmitter is 100mm away, and BATTERY powered. Does anyone know what I can do here, short of changing to 2.4GHz? |
| DaJMasta:
Take a look at your opamp's power rail. A low power transmitter typically gulps down current compared to normal operation when transmitting a packet, so it really could just be a bypassing issue. My guess is the PSU is not fast enough to respond to the output change and probably has low output capacitance. |
| exmadscientist:
LM7321 appears to have bipolar inputs (though the datasheet does not say this explicitly... how lazy does a manufacturer have to be to not indicate the input type?), which are quite susceptible to RFI rectification. There are several app notes out there describing this effect and proper solutions to it, though I'm feeling too lazy to link them at the moment (guess that was contagious... that way you can blame National....). For a quick fix you can try substituting a FET input type and seeing if things get better. |
| OwO:
Yes any RFI on any pin of the op-am can get rectified and show up as an offset. There are op-amps with specified EMI rejection (e.g. TP2311) to deal with this problem, but I recommend just designing your board to avoid any EMI getting in in the first place. At least one solid ground plane, striplines for all sensitive signals, filtering on off-board connections, etc. |
| FenTiger:
What's the bandwidth of your scope, and how are you connecting it to the power rail? If 920MHz was getting onto the rail, would it show up on the scope at all? What happens if you add a small decoupling cap directly across the op-amp power pins? |
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