difference amplifier offset trimming

[Marte Schwarz]

Hi John,

The impedances are between 100Kohm to 1Mega ohm.

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

 

[john]

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.

2. What should be the winding ratio?

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

4. 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

 

[Marte Schwarz]

Hi John,

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.

2. What should be the winding ratio?

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

4. 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?

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]

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 :-(

 

[Winfield]

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.

2. What should be the winding ratio?

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

4. 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?

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.