50mA 100ns current pulse

Thread Starter

Bakez

Joined Aug 21, 2012
31
I need to make a current pulse of 50mA for 100ns

I understand that it isn't so difficult to make a 50mV voltage pulse from a pulse generator, but to make a current pulse I need to make some kind of voltage controlled current source?

Does anyone have any tutorials on this?

I want to apply a current pulse to a 10nF capacitor and measure its ESR.

There is a circuit in this PDF here: http://cds.linear.com/docs/en/application-note/an47fa.pdf on page AN47-65 which shows an opamp voltage controlled current source that gives pulses of similar magnitude to 50mA and 100ns but it is from 1991

Surely if its possible in 1991 it is possible now in 2013?
 

shortbus

Joined Sep 30, 2009
9,441
Quote, "Surely if its possible in 1991 it is possible now in 2013?"

Sure, you just use that schematic, in the PDF. Just because this is 2013, they don't make a "new" application note, for thing's that still work.
 

Thread Starter

Bakez

Joined Aug 21, 2012
31
Yes but I would like to understand how it works specifically.

I don't think I can just apply the schematic generically to my application, I dont think one of those op-amps exists anymore
 

crutschow

Joined Mar 14, 2008
30,113
Yes but I would like to understand how it works specifically.

I don't think I can just apply the schematic generically to my application, I dont think one of those op-amps exists anymore
Both op amps show up on the Linear Technology website. Why do you think one doesn't exist?

Basically op-amp A2 senses the voltage across the current shunt resistor R and sends a signal to A1 to maintain the voltage across that resistor proportional to the input voltage. That generates a load current proportional to the input voltage.
 

crutschow

Joined Mar 14, 2008
30,113
but this lt1994 is a 'video difference amplifer' http://www.linear.com/product/LT1194

and appears to have different datasheet values than those described in the application note from the year 1991 (page an47-6)?

what is a 'video difference amplifier'?
It is a high speed differential (difference) amplifier with a fixed gain of 10 (20dB). If it has the same part number as the 1991 reference then it should still work. Any differences between the part operation between then and now should be minor.
 

Thread Starter

Bakez

Joined Aug 21, 2012
31
But I cannot get hold of these components in this circuit

You are right I dont know why I assumed they were different

I have simulated it in LTspice and it works.

TO apply a 100ns 100ma (roughly) pulse to a 20nF capacitor

However I need to build this and I cannot use these components as the lead time is about 10 weeks it seems. I need to select other similar parts from another manufacturer
 

ramancini8

Joined Jul 18, 2012
473
Jim Williams was a very good engineer because he paid attention to detail and never faked anything; thus you can use his circuits with a high level of confidence. Also, you can gate the output of a current source with a transistor gaining any PW you need.
 

Thread Starter

Bakez

Joined Aug 21, 2012
31
This type of circuit has been suggested by other manufacturers

http://www.ti.com/lit/ds/symlink/ina128.pdf - figure 7

http://www.analog.com/static/imported-files/application_notes/AN-968.pdf - 2nd page

But the opamps they use in this appear to be very low - slew rate of 5/us. I need more like 400V/us. I dont know about opamp parameters beyond slewrate really. In the simulation it appeared that if I put faster opamps for A1 then the circuit performed worse

Also as my load is more like 1ohm with 20nF capacitance I cant use his circuit exactly as I think I need some compensation for this level of capacitance
 
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