difference amplifier offset trimming

Hi Marte

The impedances are between 100Kohm to 1Mega ohm.

John

Hi John,

May be this is the resistance (DC value). A impedance is a complex (frequency dependent) value. This is significant here. above 1 kHz the absolute value of your testimpedance will be much lower, I guess. If not, then you have to give up to kilovolt. Then you need a transformer very surely. You may try out a power transformer using the "primary" (110V side) as secondary. So for a 5V transformer you only need 50 V peak from your current regulator...

This may be the reason for interrupted saturated coils I told about...

Marte

Hi,

I am confused about the following things

1. What kind of transformer should I use, power transformer, current transformer, pulse transformer . Current ( range 10nA to 1mA) is the input to the transformer (primary) and current is the output of the transformer , my load is 100kohm.

1. What should be the winding ratio?

2. Input current waveforms are arbitray, continuous, pulses ( single or many ) and brust waveforms?

1. I need the same replica of the input waveform. I suspect that the primary winding (inductor) will introduce high pass filtering, what should be done to avoid that?

Regards John

Hi John,

How would it be, that you specify your needs complete. Elsewise we allways have to read in a big crystal ball... What I know: You want to stimulate a small tissue (retina of a rat), so your electrodes may be pretty small (needle electrodes?). You expect a impedance about 100 kohm (which will be very frequency depending not only one value for all). Your current peaks will be up to 1 mA. The waveform is arbitrary. You didn't talk anything about duration repetition rate....

A few infos more please then someone may help with concrete tips.

Marte

If you are interested in my help, then please write me the specification via email. here in this group the message may reach me or not. there is too much garbage arounud :-(

John, learn transformer basics: e.g. primary voltages scale by the turns ratio, as do secondary currents. Impedance ratios are equal to the square of the turns ratios, primary load looks like secondary load times impedance ratio, in parallel with the reactance from the primary's magnetizing inductance (the secondary load impedance is of course due to the electrode skin resistance). The ratio is high at high frequencies, and can be ignored, but at low frequencies it determines the -3db cutoff frequency. To get a high magnetizing inductance you'll need more turns, as determined by the core's A_L parameter, which is specified in terms of nH/turns-squared.

Once you learn these things and begin to understand transformers, you can either design your own, or understand how to evaluate a commercial data transformer, as has been suggested. It's likely in the latter case that you'll need impedance-measuring equipment, but it's also very useful in the former case. Either way it's valuable. I have found eBay to be a good source for old laboratory-grade high-performance HP vector impedance-measuring equipment, but for simplicity, ordinary ac-attenuation measurements may suffice.