Current feedback amplifiers(op amps)

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Are there any MD circuits using these at the front end?

[Op04]

AD844 ? Never seen one

[Detectorist#1]

AD844 is very fast, but PI MD not needs so fast OpAmps. The noise of 2nV sqr Hz at 1KHz is too big for sensitive PI MD. The price of 16USD is unjustified in this case.

[Op04]

AD844 is very fast, but PI MD not needs so fast OpAmps. The noise of 2nV sqr Hz at 1KHz is too big for sensitive PI MD. The price of 16USD is unjustified in this case.

NE5534 more higher than this 4.5nV/Hz at 1khz It's cheap. Is there any opamp you would suggest?

[Detectorist#1]

Hi Op04,

My opinion is that OPA1611AID with 4nV voltage noise and 8pA current noise at 2Hz is the best OpAmp for the first stage of PI MD.

[Op04]

Hi Op04,

My opinion is that OPA1611AID with 4nV voltage noise and 8pA current noise at 2Hz is the best OpAmp for the first stage of PI MD.

It looks good thanks Detectorist#1! But..Too bad no through hole package and it doesn't have null pin. I still tooking cheaper option from AD844 AD847 which has null pin and through hole package.

[Detectorist#1]

Hi Op04,
The price of AD847JNZ (DIP8 case)
Digikey 13.51$
Element 14 14.02$
Mouser 16$

The price of OPA1611AID (SOIC-8 case)
Digikey 5.54$
Mouser 4.89$
Farnell 5.60$

AD847 haves 48nV sqr Hz noise at 10Hz and you must forget for detector with high sensitivity for small nuggets.
The SOIC-8 case isn't so hard to use with cheap SOIC-8 to DIP8 adapter - try!

OPA1611AID haves only 0.5mV input offset voltage. Even with gain of 1000, output offset voltage will be 0.5V. No need for nulling pins.

[Op04]

Hi Op04,
....
OPA1611AID haves only 0.5mV input offset voltage. Even with gain of 1000, output offset voltage will be 0.5V. No need for nulling pins.

Ok I get your point thanks. If so, we don't need trimmer for surf(R43) by using OPA1611??

[Detectorist#1]

Yes, you not needs all parts for nulling the OpAmp in the first stage of Surf metal detector using OPA1611AID. With fast search coil and some reducing of the gain of the first stage (by reducing of the value of R44), this detector is capable to find very small nuggets.

[Op04]

Yes, you not needs all parts for nulling the OpAmp in the first stage of Surf metal detector using OPA1611AID. With fast search coil and some reducing of the gain of the first stage (by reducing of the value of R44), this detector is capable to find very small nuggets.

Wow! I have to say many many thanks for this! Please tell me what your suggestion the right value for R44?

[Detectorist#1]

This depends of how fast is the used coil. Fast coil + Reducing (2-3 times) of the value of the R44 will help to processes the signals from small nuggets

[Op04]

This depends of how fast is the used coil. Fast coil + Reducing (2-3 times) of the value of the R44 will help to processes the signals from small nuggets

Ok I get it.. thanks!

[Detectorist#1]

SOP8_SOIC-8 to DIP8 PCB Adapter
Digikey 0.52Euro
Farnell 0.42P
Mouser 0.512Euro

[Op04]

SOP8_SOIC-8 to DIP8 PCB Adapter
Digikey 0.52Euro
Farnell 0.42P
Mouser 0.512Euro

Ok thanks!