From: Andreas Robinson (sleeper75se_at_yahoo.se)
Date: 2002-01-28 14:35:06
Ok, second try. The brainmaster input stage, slightly
modified, attached as a text file.
/Andreas
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>
> R6
> ___/\/\/\/\__
> C4 | |
> INPUT___| |________ | ----U---- | V+
> 1 | | | | |_| AD620 |_| |
> / | 1| |\ |8 |
> R4\ |_________|_|-\ |____|
> / 2| | \_ |7
> | _________|_|+ / |__|_______________\ OUTPUT
> C5 GND | 3| | / |6 | / STAGE 1
> INPUT___| |________| ___| |/ ref|____/|\_
> 2 | | | | 4| IC-1 |5 | |
> / | --------- | |
> R5\ V- | |
> / R7 | |
> \ ___/\/\/\/\_________| |
> | | |
> USER _________| | C6 |
> GND | |_______________| |_____|
> GND | | | |
> | ----U---- V+ |
> | __| OP-90 |__ | |
> | 1| |\ |8 | |
> |____|_|-\ |____| |
> R8 2| | \_ |7 |
> GND _____/\/\/\_____|_|+ / |__|________|
> 3| | / |6
> __| |/ |____
> | 4| IC-2 |5
> | ---------
> |
> V-
>
> Component values:
> R4: 10M 1% metal film
> R5: 10M 1% metal film
> R6: 1000 1% metal film (G = 50)
> R7 200K 1% metal film
> R8 200K 1% metal film
> C4: 0.01 uF 2% polypropylene
> C5: 0.01 uF 2% polypropylene
> C6: 0.1 uF 2% polypropylene
Connect V+ to a 3V battery and V- to -3V. Decouple with 100nF to ground, close to the IC-packages. You need four caps for that, 2 for the AD620 and 2 for the OP-90. Also decouple with a pair of 10uF capacitors, they can be shared by both IC's.
The gain is set by R6 and is calculated as R6 = 49400 / (G - 1). You get the results in ohms.
If you use a 16-bit sound card, with an input swing of +/- 1V, the bitstep of the soundcard is 30uV.
A gain of 100 should consequently be enough, as that would amplify 1uV to 100uV. Use R6 = 510 ohms, or lower for higher gain.
If you don't want the AC-coupling, don't use the OP-90 and connect pin 5 (ref) of the AD620 to ground.
The cutoff frequency for the AC-coupling is set by R7 and C6. fc = 2*pi*R7*C6 (resistance in ohms, capacitance in farads, result in Hertz). Current values gives a cutoff of 0.125Hz. If you change R7, change R8 as well.
This archive was generated by hypermail 2.1.4 : 2002-07-27 12:28:37 BST