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Old 21st July 2011, 12:48 PM   #121
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Hi George, some more measurements as requested, though I'm not sure what to make of them! Certainly not what I was expecting for the 1uF caps.

I did these on the breadboard, so it may be mucking up the measurements. perhaps I really need to solder.

the three BC's first.

single measured 0.22 or 0.21 ohms
two in series measured 2.7 ohms
three in series measured 4.4 ohms

two in parallel measured 0.02 ohms
three in parallel measured 0.02 ohms

I then tried 4 X 1uf Axons (polypropylene).

single measured 0.16 ohms
two in series 2.6 ohms
three in series 4.3 ohms
four in series 5.8 ohms

two in parallel measured 0.01 ohms
three in parallel measured 0.03 ohms
four in parallel measured 0.06 ohms

a single 3.6 uF axon measures 0.01 ohms
now the two siemens 100nf

single 17 ohms
two in series 29 ohms
2 in parallel 8.3 ohms

and finally evox rifa 100nF caps

single 16 ohms
2 in series 27 ohms
3 in series 42 ohms
4 in series 57 ohms
5 in series 73 ohms

2 in parallel 7.7 ohms
3 in parallel 4.9 ohms
4 in parallel 3.4 ohms
5 in parallel 2.7 ohms (very close to the two 1uf in series results)

and just for fun 2 X silmic II 1uF

single 2.8 ohms
in series 6.2 ohms
in parallel 0.73 ohms

I'm not going to try and make any sense of all that tonight

Tony.
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Old 3rd November 2011, 12:21 PM   #122
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Quote:
Originally Posted by gpapag View Post
I decided to build an one IC (AD620AN) Instrumentation Amplifier http://www.analog.com/static/importe...eets/AD620.pdf
AD620 is the only Instr. Amp. IC I found locally. It costs 5.3 Euro plus VAT

I housed it in an aluminum enclosure, for to do descent low level measurements on it.
If it will perform satisfactorily, it will be be used as a stand alone unit.
DC power supply is provided through a 4 pin plug.

Three gain steps selection is through an On-Off-On switch.
Below you’ll see the schematic and the first measurement
Hi George, I finally started today. I'm going to have to get myself a small aluminium box and a switch and I should be set. I think I'm going to run it off a 12V battery (same one I use for the original preamp. I'll just put a voltage divider to get +-6V I think it should be ok since the AD620 only draws 1.3mA!! I wasn't sure why so many resistors, but I could see that there were at least enough for 50 X 100 X and 1000 X gain and possibly some other values.

The only thing I'm not sure about is using two bnc's for the input + and - I'm not that familiar with using opamps, and don't really understand how that is to work. do I only use two inputs when looking at the difference between two signals or is it for balanced input? sorry for my stupid question

anyway here is a picture of the first steps familiar but not quite the same

Tony.
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Old 3rd November 2011, 09:05 PM   #123
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OK I've slept on it, and figure the way to use the two inputs is to only use the hot of each for connecting to the circuit under test, and only use the sheild on the box end. +ve one to the point of interest, -ve one to the zero volts or earth of the circuit under test. I guess this should remove the possibility of any ground loops between the DUT and the device which is taking the output.

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Old 21st November 2011, 10:10 AM   #124
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I've finished the instrumentation amp tonight I haven't actually tried it out yet, but I did perform a rudimentary health check. I hooked up the battery (which needs a bit of a charge) and checked for dc offset on the output with the multimeter set to 200mV DC. My voltage divider is not giving totally symetrical readings, but it is pretty close.

1X 0.0 mV
10 X -0.1 mV
50 X -0.8 mV
100 X -1.6mV
1000 X 16 mv

So it looks like the gain settings are pretty much spot on (except for 10X which I was expecting because my resistors didn't work out exactly to 5.49K.

It would appear that I have about -16uV offset on the input. Measurements were taken with both inputs floating.

here is a picture of the guts

Getting too late tonight to try and measure something tonight, but soon

Edit: power input is a bit dodgy in that I used a stereo 3.5mm jack, reason being I couldn't find an insulated dc socket (I need to keep the -ve of the battery floating and the virtual zero volts tied to the case).

Tony.
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Last edited by wintermute; 21st November 2011 at 10:12 AM. Reason: comment on dodgy power socket.
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Old 29th November 2011, 08:19 PM   #125
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Hi Tony
Sorry for my delayed answer.
You've done a great construction. A very good alum. housing too.
About your question on how to connect the two inputs, you are correct. You can use the "differential" mode , as well as the "single ended" connection (see attached sketch).
Have a look at the AD 620 data sheet. If I remember well, the (-) Input is less noisy than the (+) Input. If this is so, the (-) Input should be connected to the weaker signal when you use the "differential" connection When you use the "single ended" connection, the (-) Input should be connected to the "hot" signal and the (+) Input to the GND.

Keep on working (and walking )

Regards
George
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Last edited by gpapag; 29th November 2011 at 08:22 PM.
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Old 30th November 2011, 12:37 AM   #126
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Thanks George!
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