Target response tester

[Ferric Toes]

My thought for checking decay slope. The dashed line is the calculated slope. The input on the LOG114 is 10k instead of 5k in the picture. Use 100p capacitor for 1usec TC, 330p for 3.3usec TC and 1n for 10usec TC.

Log amp circuits that I have seen have a much lower input resistor that you have. 100 - 500 ohms is common.

Eric.

[Ferric Toes]

I'm now using the Picoscope 2206B for object tests but there is still a lot to learn about it's capabilities. I haven't yet got a log function on the Y channel yet, but I am building my own to go between the test rig and the scope.
The plot here shows the bottom end of the decay of a Silver Dollar where the signal disappears into the noise at 0.9mS. This is with a TX width of 1.0mS. I took a point where the curve reached 600mV as the starting voltage, then took 37% of that as the next point of 222mV which related to 128uS; which in turn is 1TC. The next point to measure is when the amplitude is 14% which is measured in the next graph at 83.67mV and 261uS. 261/2 = 130.5uS which is close enough to 128uS for the present purpose. Graph 1 gives the period of of 1TC and graph 2 the period of 2TC.

The top of the graph that goes off scale extends to 3.5V, which is the saturation level of the two stage preamp which consists of a NE3354 x10 gain, then a NE5532 giving plus and minus outputs with a gain of x49.

Interestingly, if I substitute a Nickel with a copper core, there is no measurable shift in TC from that of the silver one. When doing decay plots recently on a Bitscope it appeared that there was a difference. I will have to check that, as maybe the differences are at earlier times due to 'skin effect' of the nickel plating.

Eric.

[green]

My thought for checking decay slope. The dashed line is the calculated slope. The input on the LOG114 is 10k instead of 5k in the picture. Use 100p capacitor for 1usec TC, 330p for 3.3usec TC and 1n for 10usec TC.

[green]

Do you have any thoughts on why the errors occur when the input is low, ie. below 25mV ?
I was thinking it's simply a DC offset, that becomes more obvious as the actual DC signal level falls off.
But I did wonder if it was due to a slower log-amp response at lower input currents.
I guess ideally you want to drive the log-amp hard, so that these errors become less significant.

And regarding my accelerometer build, it looked surprisingly similar to your logamp, I stuck it on some copper-clad board, knife-cut with a simple pad layout. I'll try and find the pics I took of it.

I would assume the 40mm square can target would give the most accurate TC value (it's dimensions will be more precise, the longer TC is easier to measure accurately), and the other smaller samples should be in direct proportion. The fact that they're slightly high is curious. A more thorough experiment would be to test many (8, 10?) different size samples, and plot them on a 'straight-line' graph.

I think it's either speed or the no target signal being not quite zero early in the decay like you suggested. Going to try to discharge a capacitor into the input with discharge TC of 1, 3.3 and 10usec time constant.

Charted the silver quarter again. The 300 and 600usec Tx width's with constant rate looked different than constant current with the scope traces I posted awhile back. Wanted to retest and chart to see if the same thing happened. The silver quarter has a TC around 150 to 160usec. Any thought why the 300 and 600usec Tx width is about the same constant current but different constant rate?

[Skippy]

Do you have any thoughts on why the errors occur when the input is low, ie. below 25mV ?
I was thinking it's simply a DC offset, that becomes more obvious as the actual DC signal level falls off.
But I did wonder if it was due to a slower log-amp response at lower input currents.
I guess ideally you want to drive the log-amp hard, so that these errors become less significant.

And regarding my accelerometer build, it looked surprisingly similar to your logamp, I stuck it on some copper-clad board, knife-cut with a simple pad layout. I'll try and find the pics I took of it.

I would assume the 40mm square can target would give the most accurate TC value (it's dimensions will be more precise, the longer TC is easier to measure accurately), and the other smaller samples should be in direct proportion. The fact that they're slightly high is curious. A more thorough experiment would be to test many (8, 10?) different size samples, and plot them on a 'straight-line' graph.

[green]

Some targets cut from an aluminum can side, LOG114 output. Looks alright above 25mv input. Not sure the slopes are correct, need to check decay response with known input. Maybe something similar to spice model, discharge a capacitor into the input.

[green]

I was going to ask about your intended LOG114 circuit, and wondered if you were going to simulate it in Spice first, as you have the skills. I haven't read the datasheet for the IC, (though I did download it) but I was wondering how best to get round the 0.5V offset for the intended photodiode bias.

How did you wire up the IC ? I've just recently wired up an IC accelerometer in a tiny leadless package, some patience is needed.

How did you wire yours? I wired an IC accelerometer in a tiny leadless package at work a few years back without the adapter. Was thinking might have to make modifications to the wiring this time so I used the adapter so I didn't have to rewire to the log amp.

[green]

Some scope traces with a silver quarter at 3 different Tx width's, 1/2A peak constant current and 1A peak constant rate. Probably easier to see difference if they were on the same chart.

[green]

I was going to ask about your intended LOG114 circuit, and wondered if you were going to simulate it in Spice first, as you have the skills. I haven't read the datasheet for the IC, (though I did download it) but I was wondering how best to get round the 0.5V offset for the intended photodiode bias.

How did you wire up the IC ? I've just recently wired up an IC accelerometer in a tiny leadless package, some patience is needed.

Didn't try LOG114 in spice because I haven't earned how to add other spice models to LT spice. The TRT circuit is the first one I've put together without having to modify the circuit or fix a wiring error in a long time, maybe the first time. Could attach schematic if anyone is interested.

Including a picture of how I wired the LOG114. Pin numbers are reversed, pin16 adaptor is pin1 of LOG114. Go clock wise, pin2 LOG114 not connected. Messed up pin 2 adaptor but it didn't matter because I don't use that op amp.

[6666]

Looking at your reply again I see I didn't explain may test method very well. I adjust ground balance(target sample time) for no change at integrator out when I place the bag of dirt on the coil or take it off, similar to pumping the coil. Most of the time distance measurements are made with the dirt off, GB on or off. I have checked distance measurement with the bag of dirt on and off the coil. The bag of dirt on coil didn't seem to effect the signal change vs distance measurement.

Thanks for the clarification

[Skippy]

I was going to ask about your intended LOG114 circuit, and wondered if you were going to simulate it in Spice first, as you have the skills. I haven't read the datasheet for the IC, (though I did download it) but I was wondering how best to get round the 0.5V offset for the intended photodiode bias.

How did you wire up the IC ? I've just recently wired up an IC accelerometer in a tiny leadless package, some patience is needed.

[green]

Thanks for reply, had not thought of retesting with dirt off.

Looking at your reply again I see I didn't explain may test method very well. I adjust ground balance(target sample time) for no change at integrator out when I place the bag of dirt on the coil or take it off, similar to pumping the coil. Most of the time distance measurements are made with the dirt off, GB on or off. I have checked distance measurement with the bag of dirt on and off the coil. The bag of dirt on coil didn't seem to effect the signal change vs distance measurement.

[green]

Added the log amplifier. Glad I had only 1 chip that size to hard wire. Want to thank those that made suggestions when I was trying to learn something about log amps. I decided on the LOG114 because of speed and it would work with the +-2.5v power supply that I use for the other analog circuits. Did a comparison test with the log amp vs the way I normally do it. The two traces the way I normally do it didn't line up but the slope was the same. The 1 layer decays about 1 decade every 4usec both methods. TC=(time/decade)/2.3, =1.7usec. The 4 layer decays about 1 decade every 14usec both methods. TC=(time/decade)/2.3, =6.1usec.


Log amp looking at pre amplifier out, gain about 450 vs (target sample recording-no target recording) (CH1)pre amplifier out, gain about 450 (CH2)amplifier out, gain 9 total gain about 4000. Both methods similar results. Both methods about 2 decades limited by amplifier noise. Log amp, real time the other takes a lot longer. Log amp needs a good no target zero, other method charts difference so it cancels any offset.

[6666]

dirt on or dirt off coil.

Thanks for reply, had not thought of retesting with dirt off.

[green]

Hi Green , sorry to put your thread slightly off track, but I was wondering if you could confirm something for me please, back when you were doing your ground balance experiments to eliminate your bags of dirt, were you simply putting the coil on top of the bags of dirt or were you bobbing the coil up and down ? thanks.

The coil is stationary, bottom up. The bag of dirt is put on bottom of coil. The ground balance is adjusted for no change integrator out, dirt on or dirt off coil.