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How to measure bi-directional small signal currents

P

Pubudu

Jan 1, 1970
0
Hi All,

I want to measure small current pulses (For applied voltage pulses)
in the range of 10 to 300 uA. Will you have any idea on that ?. The
frequency of the pulses will not exceed 5 kHz and the amplitude is
between 1 to 3 V Maximum. I think differential amplifier is fine in
this case but I want to make sure before desinging the circuit. I can
measure the corresponding voltage with a NI card and convert that back
to current.

Pubudu.
 
L

Luhan

Jan 1, 1970
0
Pubudu said:
Hi All,

I want to measure small current pulses (For applied voltage pulses)
in the range of 10 to 300 uA. Will you have any idea on that ?. The
frequency of the pulses will not exceed 5 kHz and the amplitude is
between 1 to 3 V Maximum. I think differential amplifier is fine in
this case but I want to make sure before desinging the circuit. I can
measure the corresponding voltage with a NI card and convert that back
to current.

So, what is this about? Battery charger? Your post seems to have
"lost something in translation". Its hard to tell just what you are
asking for here.

Anybody here got a clue?

Luhan
 
W

Winfield Hill

Jan 1, 1970
0
Luhan wrote...
So, what is this about? Battery charger? Your post seems to have
"lost something in translation". Its hard to tell just what you
are asking for here. Anybody here got a clue?

I think Pubudu has just been a bit careless with his description.
For example, is he applying a stiff voltage source and seeking the
resulting current into the load?* Or is he applying small current
pulses as he says, and wants to measure the voltage drop? Or is he
perhaps applying some hybrid between current and voltage pulses?

* These techniques can go under the heading "source-measure" and
are a lead-in to a field of interesting instruments and circuits.
 
B

Ban

Jan 1, 1970
0
Pubudu said:
Hi All,

I want to measure small current pulses (For applied voltage pulses)
in the range of 10 to 300 uA. Will you have any idea on that ?. The
frequency of the pulses will not exceed 5 kHz and the amplitude is
between 1 to 3 V Maximum. I think differential amplifier is fine in
this case but I want to make sure before desinging the circuit. I can
measure the corresponding voltage with a NI card and convert that back
to current.

Pubudu.

I also don't understand your question exactly, but maybe you want something
like this. This array of opamps translates a current into voltage and has
zero voltage drop, so you can measure without any influence and also on the
high side as long as the compliance is met.
___
+-|___|------+
| R |
I | |\ |
o----->----+-|+\ | ___ ___
| >--+----)-+-|___|-+-|___|-+
| +-|-/ | | | R1 | R1 |
| | |/ .-. | | | ===
| | | | | | | GND
| | | |R | | |
| | '-' | | | |\
Ue=0 | | | | +-|+\ Uout
| +-------+ | | | >--+----o
| | | | | +-|-/ |
| | .-. | | | |/ | Uout=2*R*I
| | | | | | | |
| | | |R | | | |
| | |\ '-' | | | |
| +-|-\ | | | ___ | ___ |
V | >--+----+-)-|___|-+-|___|-+
o-----<----+-|+/ | R1 R1
I | |/ |
| ___ |
+-|___|--------+
R
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)
 
P

Pubudu

Jan 1, 1970
0
Ban said:
I also don't understand your question exactly, but maybe you want something
like this. This array of opamps translates a current into voltage and has
zero voltage drop, so you can measure without any influence and also on the
high side as long as the compliance is met.
___
+-|___|------+
| R |
I | |\ |
o----->----+-|+\ | ___ ___
| >--+----)-+-|___|-+-|___|-+
| +-|-/ | | | R1 | R1 |
| | |/ .-. | | | ===
| | | | | | | GND
| | | |R | | |
| | '-' | | | |\
Ue=0 | | | | +-|+\ Uout
| +-------+ | | | >--+----o
| | | | | +-|-/ |
| | .-. | | | |/ | Uout=2*R*I
| | | | | | | |
| | | |R | | | |
| | |\ '-' | | | |
| +-|-\ | | | ___ | ___ |
V | >--+----+-)-|___|-+-|___|-+
o-----<----+-|+/ | R1 R1
I | |/ |
| ___ |
+-|___|--------+
R
(created by AACircuit v1.28 beta 10/06/04 www.tech-chat.de)


Hi GUys,

Sorry for my writting and I really appreciate your response.
My problem is this.
I am applying a oscilating voltage pulses to a chemical cell and I want
to study the current flow through cell. Voltage pulse frequency is
between 1 Hz to 5 kHz and expected current is in between 10 to 300 uA.
(Cell impedance is between 10kOhms and 100 kOhns approximately)
Voltage signal is applied as like this (using two analog channels in NI
card):
First pulse : Ch0 = 1 V and Ch1 = 3 V.
Second pulse Ch0 = 3 V and Ch1 = 1 V
Third pulse : Ch0 = 1 V and Ch1 = 3 V.
Fourth pulse Ch0 = 3 V and Ch1 = 1 V
Fifth pulse : Ch0 = 1 V and Ch1 = 3 V so on . . . . . .

Ch0 and Ch1 are connected to the two cell terminals.
 
B

Ban

Jan 1, 1970
0
Pubudu said:
Hi GUys,

Sorry for my writting and I really appreciate your response.
My problem is this.
I am applying a oscilating voltage pulses to a chemical cell and I
want to study the current flow through cell. Voltage pulse frequency
is between 1 Hz to 5 kHz and expected current is in between 10 to 300
uA. (Cell impedance is between 10kOhms and 100 kOhns approximately)
Voltage signal is applied as like this (using two analog channels in
NI card):
First pulse : Ch0 = 1 V and Ch1 = 3 V.
Second pulse Ch0 = 3 V and Ch1 = 1 V
Third pulse : Ch0 = 1 V and Ch1 = 3 V.
Fourth pulse Ch0 = 3 V and Ch1 = 1 V
Fifth pulse : Ch0 = 1 V and Ch1 = 3 V so on . . . . . .

Ch0 and Ch1 are connected to the two cell terminals.

Than I think this is exactly what you will need. If you use a current shunt,
it will have a voltage drop and to have a good sensitivity the shunt has to
be quite large. So the voltage is not constant, but varies. With high
frequencies there will be also a lowpass behaviour distorting the pulses.
this thing has zero impedance, so the upper frequency is much higher and
pulses are not distorted. OP27 for the front-end and some INA as a
differential amplifier are recommended.
 
P

Pubudu

Jan 1, 1970
0
Ban said:
Than I think this is exactly what you will need. If you use a current shunt,
it will have a voltage drop and to have a good sensitivity the shunt has to
be quite large. So the voltage is not constant, but varies. With high
frequencies there will be also a lowpass behaviour distorting the pulses.
this thing has zero impedance, so the upper frequency is much higher and
pulses are not distorted. OP27 for the front-end and some INA as a
differential amplifier are recommended.




Hi Ciao Ban
I really appreciate your reply. Thank you very much. I will go head
and make it.
Again thank you very much.


Pubudu
 
T

Tony Williams

Jan 1, 1970
0
Voltage pulse frequency is between 1 Hz to 5 kHz and expected
current is in between 10 to 300 uA. (Cell impedance is between
10kOhms and 100 kOhns approximately) Voltage signal is applied as
like this (using two analog channels in NI card): First pulse :
Ch0 = 1 V and Ch1 = 3 V. Second pulse Ch0 = 3 V and Ch1 = 1 V
Third pulse : Ch0 = 1 V and Ch1 = 3 V. Fourth pulse Ch0 = 3 V and
Ch1 = 1 V Fifth pulse : Ch0 = 1 V and Ch1 = 3 V so on . . . . . .

If those pulses are square pulses then the common mode voltage
could have significant components up to at least 5x5KHz.
Getting a good common mode rejection at high frequency can be
difficult and it is always worth while investigating the
possibility of not having a CMV in the first place.

It looks like you have a two-terminal cell, which needs a +/-
2V drive and have perhaps chosen those +1V to +3V differential
signals because your NI card does not do bipolar voltages.

The circuit below will provide a +/- 2V drive to the cell
from your NI card, and produce a grounded I-V conversion.


3 _ R1 R2
_| |_---/\/\---+-/\/\--+ All R's = 10k, 0.1% (say).
1 | |
V1 +--|-\ | OP amps powered from +/-
| >-+ supply rails.
0v--|+/ |
+------------------------>ADC-3
R3 | R7
V2 +-/\/\--+- --/\/\-- -+
3_ _ R4 | R5 | R6
|_| ---/\/\---+-/\/\--+----[CELL]---+-/\/\--+-->ADC-1
1 | | | |
+--|-\ | +--|-\ |
| >-+<+/-2V | >-+
0v--|+/ | 0v--|+/
+------------------------>ADC-2

The left hand end of the cell has the +/- 2V drive on it.
The right hand is at 0V, into the virtual earth of an I-V
converter. R6=10k gives ADC-1 a scaling of 3V/300uA.
That is a bipolar output. R7 is an optional resistor
that biasses ADC-1 into being positive-only. The amount
of bias varies according to where V1 is, but this can
be easily dealt with in the software. An ADC-3 input
is useful for measuring the bias.

ADC-2 is an optional measurement that gives the exact
voltage on the cell during each pulse-polarity. Useful
for (say) software-adjusting V1/V2 for zero dc-bias
on electrochemical cells.
 
F

Fred Bloggs

Jan 1, 1970
0
Winfield said:
Ban wrote...



That's a cute circuit (nice drawing, too). Where'd you find it?

Does a configuration of back-to-back NIC inverting current mirrors with
differential pick-off ring a bell? Although analytically the same,
cross-coupling the feedback must yield synchronization advantages:

View in a fixed-width font such as
Courier.
 
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