Beachmaster Digital

[jeep6789]

I need to get the level of level A, please help，Thankyou

[jeep6789]

Hello! I want to make a wireless detector with Bluetooth and single-chip mechanism, because the op amp is required to ? 5V level, single-chip microcomputer and Bluetooth 3.3V, then in this circuit, how should I generate ? 5V and 3.3V?

[jeep6789]

If the black and non-ferrous metal is distinguished by the slope, the coil of 40 cm or less in diameter can be done. The smaller the coil, the stronger the ability to distinguish, the greater the coil, the smaller the ability to distinguish, for one meter or more

[Olly]

stb14mn50n What kind of advantage is it compared to IRF740?

The STB14NM50 is a higher voltage device (500V vs 400V) than the IRF740 and is surface mount which is why I chose it.
Initially it appears to have better Coss then the IRF740, but this is quoted at 50V and the IRF740 is quoted at 25V so they probably end up being very similar.

[Olly]

How to ensure that the 6200 output must be higher than the GND level, so that ADC can detect positive voltages.

Please read Post #38

[jeep6789]

stb14mn50n What kind of advantage is it compared to IRF740?

[jeep6789]

How to ensure that the 6200 output must be higher than the GND level, so that ADC can detect positive voltages.

[Olly]

Placement getting there, some re-arranging of the ADC and preamp to improve the analogue and digital GND plane spilt.

[waltr]

[ATTACH]57069[/ATTACH]

D3 is backwards....

[Olly]

[ATTACH]57069[/ATTACH]

Hi Jeep, actually your circuit will clamp the output permanently to around -4.5V, but I get where you're coming from...
Could I have just used a resistor-diode clamp at the output before feeding the ADC ?

The answer is yes, it might have worked if we could use a sufficiently high value resistor so that the opamp doesn't have to provide excess current when clamping.
However, the ADC needs a low input impedance and the recommended value is 22 Ohms, this would have made our clamping current 4.5/22 which is over 200mA and clearly not a viable solution.

I then tried a typical precision rectifier circuit at the output of U9 but found that the recommended filter capacitor (C2 of 1.7nF slowed the response down to unacceptable levels. Ref below: -
https://en.wikipedia.org/wiki/Precision_rectifier

What I eventually settled on was to use diodes D2 and D3 to provide unity gain for the half cycle I wasn't interested in and so prevent the output from U9 from ever going much below 0.5V. This retains the speed I need and solves the clamping current problem.

Here is a simulation showing the voltages of interest.

The top trace shows the flyback voltage
The middle trace shows the output from U8 and the bottom trace shows the final signal delivered to the ADC at T10.

Regards