difference amplifier offset trimming

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 tested the circuit for Leakage current and with zero input voltage , load resistor of value 100kohm, R = 50kohm and rail voltages of +/-

15volts, I am getting current in the range of 1 to 10 micro amp, which is undesirable for my application. The questions are as follows

1. Do I need offset trimming like the data sheet discussed it on page number 9?

1. Is there any other way to stop this leakage current?

The desirable leakage current range is less than 1 micro amp.

Please advice!

John

figure 2 ?

martin

How long would it take to solder in a trimpot and find out?

martin

john

I calculate that your measured output "leakage" current should be less than 0.075nA, based on the maximum offset voltages of the ina133 of 0.45mV, and the opa131 of 0.75mV, summed and divided by 50k, plus 0.05nA maximum bias current from the opa131. Solve that big discrepancy, before thinking about zero-trimming circuits.

Hi,

I connected the load of 100kohm resistor at the output and measured the voltage across the resistor using voltmeter. Is it a good method?

John

Is everything well shielded? Do you have AC pickup (75pA is a pretty weak current, implying high-Z, subject to pickup and rectification)? Have you looked at the various signals with a scope probe? It's not the "schematic" part of a circuit that can get you into trouble, as much as the parasitic part.

Hi,

What if the 50Kohm resistor is replaced by 5kohm resistor? will the leakage current go higher?

John

The offset-voltage contribution will be 10x higher, yes, but it's still under 240nA, low compared to your value.

Hi, I tested the circuitry again and I am getting the leakage current with

5kohm resistor of approx. 240nA. Is there any way that I reduce it less than 0.075nA.

John

You can program up to 1 or 2 mA with a 5k resistor, and the 0.24uA zero-current error you mention observing is 1/8000 of that, which is pretty good. If you add a little zeroing (which can be on the input-voltage side) you may be able to improve that by 10x or more. This gets you into the 10ppm area, but the matching errors of the difference amplifier may also begin to cause trouble and need attention.

If you want 75pA zero accuracy in a 2mA full-scale circuit, that's only 0.04 ppm. To attempt to achieve that you'd need an entirely different approach, a true current source made with precision electronics, and perhaps with range switching, etc.

What are you trying to do?

Hi,

If you want 75pA zero accuracy in a 2mA full-scale circuit, that's only 0.04 ppm. To attempt to achieve that you'd need an entirely different approach, a true current source made with precision electronics, and perhaps with range switching, etc.

So, the current source that I am using is not a true current source. Would you advice that what is a true current source and how can I build it?

What are you trying to do? I am trying to build a system that can stimulate Lab. rat's retinal tissues. Thats why I am concerned about leakage current.

Thanks John

Those that use the collector of a transistor or the drain of a FET for the output current, with an opamp providing the programming control.

Aha! Then do you need really 2mA fullscale? If you use a higher resistor value then offset voltage is less of an issue. Maybe you can have a range-switching scheme?

But I wonder if your concern is over leakage to ground, which could be eliminated with a battery-operated current source?

Hi Winfield,

No, in long term stimulation there would be a problem with dc leakage current, too. Like in all good TENS the OP should stimulate biphasic and dc-free. This is easyly done tith a output transformer.

Marte

Hi,

Can you advice any circuit diagram or FET ?

How a battery operated current source can not have a leakage current?

John

Can you advice the transformer setup? Will the transformer be able to handle frequencies in the range of 1kHz to 40Khz. current amplitudes between 1uA to 1mA?

John

I like the transformer suggestion. If you feed it with a current source, the output will also be a current but without any DC component. A 1 to 40kHz range is easy, you can get three or even four decades of frequency range if necessary. There is little practical low limit to the currents that can be handled.

Hi John,

With such "easy" things you can use simple audio transformers. May be ISDN transformer will work too. just try out. This is not a big thing. If you want to do this work "science" like, then go to a manufacurer's site and use their transformer calculation tool for their cores.

Marte

Hi Winfield,

That ist the typical way to do this in transcutan stimulation (TENS). Other way is to couple out via capacitor. But a transformer is prefereable. Often it is seen in a saturated coil design. Just interrupting a few mA flow through a primary coil and plug the secondary winding to patient. But this one is not prefereable for use in battery powered devices.

Marte

Hi,

I found the following transformer

What do you think about it... Is it appropiate to pass peak to peak

1nA to 1mA of AC current. I am also confused aboout grounding the primary and secondary of the transformer. Would they have same ground?

John