Author Topic: EEVblog #1328 - uCurrent OP189 Measurements  (Read 3962 times)

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Online Kleinstein

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #25 on: August 16, 2020, 12:36:04 pm »
Using 2 x 2 AA is usually better than the virtual ground from the OP. It need less power and is usually lower impedance. The LMV321 allows to use just 1 coin cell or could work with 5 V from an LDO. So one does not absolutely need to remove the LMV321 if the OPA189 is used.
The MCP6H01 would allow using a 9 V block.
 
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Offline Gandalf_Sr

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #26 on: August 16, 2020, 01:36:44 pm »
Using 2 x 2 AA is usually better than the virtual ground from the OP. It need less power and is usually lower impedance. The LMV321 allows to use just 1 coin cell or could work with 5 V from an LDO. So one does not absolutely need to remove the LMV321 if the OPA189 is used.
The MCP6H01 would allow using a 9 V block.
Thanks.  The MCP6H01 is good up to 16V (+/-8) but the TPS3809 voltage regulator used for the LED is rated at an absolute max of 7V so either we stay under 7V or that has to change too.

[EDIT] With 2 x (2xAAA) alkalines, the voltage would range from 6.4 down to 4.5 when the batteries were almost completely dead.  Although the MCP6H01 would work as a replacement for the LMV321, using a split battery supply means U2 and associated resistors could go away.

The TPS3809 could stay to give controlled battery level warning (LED goes out when battery is failing) but we would want the 4.55V version, part number TPS3809I50DBVR and we'd also want to change the value of the LED dropper resistor.

OK, I've ordered the parts from Digikey, I'll report back on the progress.
« Last Edit: August 17, 2020, 02:21:33 pm by Gandalf_Sr »
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Offline Gandalf_Sr

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #27 on: September 04, 2020, 04:07:10 pm »
Well, I was about to perform this upgrade today when I realized that, if moving to a split rail supply using 2+2 AAA batteries, I'm going to have to figure out a way to switch both the +3V and the -3V rails or there will be a half-powered circuit drawing current all the time.  Either that or maybe I should find a replacement for the LMV321 that can handle 4 x AAAs like Dave suggests.
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Offline Gandalf_Sr

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #28 on: September 05, 2020, 01:39:21 pm »
I figured it all out.  I did away with the short functionality which allowed me to switch the +3V and the -3V lines at the same time.  Here's what I did:
1. Used a 4 x AAA battery holder and added an extra wire (green) that's the center tap of the 4 batteries.
2. Cut away the ridges inside the plastic case using a chisel modelling knife and Dremel - the battery box just fits inside.
3. Removed U2, R6,R7,R10,C2
4. Replaced U1 and U4 with OP189IDBVRCT
5. Replaced U3 with TPS3809I50DBVR (4.55) volt version and changed R4 to 909 Ohms
6. Removed battery holder and cut tracks on PCB to remove shorting connection (see picture)
7. Soldered wires onto PCB (see picture)

On testing, I was able to set up a 0.5 uA current through the uCurrent and see 500 mV on the output (1mV/nA range) with my calibrated 34461A.  It looks like it was all successful.
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Offline apoorv3in

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #29 on: September 07, 2020, 02:52:57 am »
Modified the uCurrent based on V3 with OPA189 and LM321MF
running now with 9v Battery
[attach=1]

 

Offline EEVblog

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #30 on: September 07, 2020, 03:14:53 am »
I figured it all out.  I did away with the short functionality which allowed me to switch the +3V and the -3V lines at the same time.  Here's what I did:
1. Used a 4 x AAA battery holder and added an extra wire (green) that's the center tap of the 4 batteries.
2. Cut away the ridges inside the plastic case using a chisel modelling knife and Dremel - the battery box just fits inside.
3. Removed U2, R6,R7,R10,C2
4. Replaced U1 and U4 with OP189IDBVRCT
5. Replaced U3 with TPS3809I50DBVR (4.55) volt version and changed R4 to 909 Ohms
6. Removed battery holder and cut tracks on PCB to remove shorting connection (see picture)
7. Soldered wires onto PCB (see picture)

On testing, I was able to set up a 0.5 uA current through the uCurrent and see 500 mV on the output (1mV/nA range) with my calibrated 34461A.  It looks like it was all successful.

Have you done a full frequency sweep?
 

Offline Gandalf_Sr

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #31 on: September 09, 2020, 10:32:50 pm »
No, not yet Dave. Any tips on how to set that up, say with a Rigol MSO5074?
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Offline SilverSolder

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #32 on: September 10, 2020, 03:22:00 am »
No, not yet Dave. Any tips on how to set that up, say with a Rigol MSO5074?

 

Online Andreas

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Re: EEVblog #1328 - uCurrent OP189 Measurements
« Reply #33 on: September 10, 2020, 03:09:24 pm »
Hmm,

shure that the OPA189 is the right candidate for a (high impedant) current source input.
My measurements give a relative high (average) input bias current.
with a 10K source resistance I get a 23 uV offset voltage or 2.3 nA effective input bias current.

see also here:
https://www.eevblog.com/forum/metrology/emi-measurements-of-a-volt-nut/msg2819014/#msg2819014

with best regards

Andreas
 
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