# Constant current sources and capacitance

Discussion in 'Electronic Design' started by john, Nov 14, 2008.

1. ### johnGuest

Hello,

I am using texas instrument's part number INA2133U and OPA 131 to
build the constant current source. The following link will take you
to
the circuit and data sheet ( page 13, figure 16). The inverting input
of the amplifier is grounded.

I used 10uF capacitor in series with the resistor R to stop DC leakage
current and my load has an impedance in the range of 10Kohm to
500kohm. My question is there is a chance that the capacitance offered
by the capacitor and the load will stop the current source to shut off
and there will be no biphasic wave will pass through the capacitor.

The biphasic waves are bipolar square waves of frequencies between
200Hz to 500kHz. Will it better to minimize the DC leakage using some
trimming scheme at the INA2133 or capacitor will be ok?

If such thing capacitance exsists acorss the current source than what
will be its value in my circuit and how can I measure it?

The value of R is 50KOhm. The Max Dac voltage is +/- 2. 5 volts peak
to peak.

Thanks
Regards,

John

2. ### johnGuest

The capacitor is in series with the load.

John

3. ### Rich GriseGuest

No. What happens is the capacitor blocks our DC output, charges up, and
stops at the rail.

What ever possessed you to put a capacitor in series with a DC output?

Good Luck!
Rich

4. ### johnGuest

Hi,

So, you are saying that even if the capacitor is fully charged, it
will not only stop DC but also will not let the ac or biphasic square
wave pass through it..It will block
DC and AC both.

Will it be good idea to add a resistor at the input of the capacitor
and ground it. The resistor will stop the capacitor to get fully
charged.

Thanks
John