Author Topic: Op Amp (LMV321) Operating Near Rail Voltage - not working  (Read 1118 times)

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Offline Gandalf_SrTopic starter

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Op Amp (LMV321) Operating Near Rail Voltage - not working
« on: November 03, 2019, 02:13:05 pm »
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.
« Last Edit: November 03, 2019, 02:37:59 pm by Gandalf_Sr »
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Offline China NewBoy

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Re: Op Amp (LMV321) Operating Near Rail Voltage - not working
« Reply #1 on: November 03, 2019, 02:43:56 pm »
did you measured the opA's Vcc?
 

Offline Gandalf_SrTopic starter

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Re: Op Amp (LMV321) Operating Near Rail Voltage - not working
« Reply #2 on: November 03, 2019, 03:00:53 pm »
did you measured the opA's Vcc?
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)
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Offline Zero999

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Re: Op Amp (LMV321) Operating Near Rail Voltage - not working
« Reply #3 on: November 03, 2019, 07:30:26 pm »
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
 
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Offline Gandalf_SrTopic starter

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Re: Op Amp (LMV321) Operating Near Rail Voltage - not working
« Reply #4 on: November 03, 2019, 09:42:29 pm »
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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Offline Gandalf_SrTopic starter

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Re: Op Amp (LMV321) Operating Near Rail Voltage - not working
« Reply #5 on: November 09, 2019, 02:13:40 pm »
So I dropped in the TLV313-Q1 to replace the LMV321 (same footprint and pinout) and all works as expected now  ;D
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