Electronics > Projects, Designs, and Technical Stuff
Op Amp (LMV321) Operating Near Rail Voltage - not working
Gandalf_Sr:
See the circuit in the picture...
The main GPS module (top right) sends 3.3V (3.287V measured) out of VccRF and that is fed back into V_Ant via a 10 \$\Omega\$ resistor (R2). The 3.3V from VccRF also goes across a potential divider (R9+R10) which feeds a voltage of approx 3.1 V to the + input of U1.1, the LMV321 Op Amp.
If the antenna is connected it pulls approx 25 mA and there's a voltage drop across R2 and I measure 3.057 V on the - input of U1.1. If the antenna is disconnected, there's no voltage drop across R2 and I measure 3.287 V on the - input of U1.1.
Here's the problem; the output of U1.1 (pin 4) stays at 60 mV all the time (it does so on 2 PCBs I have made and tested) but my logic says that, because the - input of the Op Amp changes from above to below the voltage on the + input, I should see the output swing from rail to rail.
Is there something wrong with my understanding on how this should work? I adapted the circuit from fig 14 of the Ublox MAX-8/MAX-M8 Hardware Integration Manual where they use a LT6000 Op Amp and R2 is 560 \$\Omega\$ . I have tried a 560 \$\Omega\$ resistor but it doesn't change the behavior. Is the LT6000 a way better Op Amp or can you guys suggest an alternative if the issue is my cheap Op Amp selection?
Thanks in advance.
[EDIT1] I think it's because the LMV321 only works for input voltages up to Vcc - 1V while the LT6000 allows inputs all the way up to Vcc, if there's alternatives, I need a device in the SC-70-5 package.
China NewBoy:
did you measured the opA's Vcc?
Gandalf_Sr:
--- Quote from: China NewBoy on November 03, 2019, 02:43:56 pm ---did you measured the opA's Vcc?
--- End quote ---
Yes, it was 3.287 Volts.
I think I've found a replacement, Texas Instrument's TLV313-Q1 which features RRIO (rail to rail input output)
Zero999:
Yes, I think you're right: The LMV321's input common mode range only extends to 1V less than the positive supply.
There are plenty of op-amps with rail-to-rail inputs. The TLV313 is a sensible choice. Also try the MCP6021.
http://ww1.microchip.com/downloads/en/devicedoc/20001685e.pdf
Gandalf_Sr:
It seems that the MCP6021 doesn't have the same pinout as the LMV321 but the TLV313-Q1 does. I have 75 PCBs made.
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