thank you bro. That means, should I try more than one alternative and then see which one is closest?uA709 uses external compensation, suitable replacement depends on application parameters.
Mr Brian Thank you, important note and good advice. I will see what can be changedBrian.
thank you for your response brother....In the sniffer xr 71 (delta pulse 2) device, the LF357 circuit was used. But there is a difference between the two schematicsMust confess that I didn't try to analyze circuit operation in detail, see that we have a kind of a synchronous rectifier following uA709 gain stage, thus harmonics of 500 Hz pulse frequency may play a role as they translate into phase shift. Can be that 11 kHz bandwidth if TL071 is sufficient. I would probably try decompensated OP like LF357 (20 MHz GBW).
(( I think the idea that is can distinguish gold from other non-ferrous metals is hard to believe ))To be honest FvM, I doubt it works well at all anyway. It seems to measure change in inductance in the pickup coil by pulsing it with high current via the IRF740 and measuring the resulting voltage across it through the uA709. At first I thought it might be looking for residual resonance in whatever it detected, a bit like TDR but that would require the uA709 to be gated off during the pulse transmission. Given the pulses come from a 555 timer running at about 450Hz, even with the relatively high gate current, I wouldn't expect a wide bandwidth is required. There are better ways of detecting metals and seeing if they are ferrous or not. I think the idea that is can distinguish gold from other non-ferrous metals is hard to believe.
Brian.
I have used the LF357 circuit in other devices, but the results are very different depending on the manufacturer.....Must confess that I didn't try to analyze circuit operation in detail, see that we have a kind of a synchronous rectifier following uA709 gain stage, thus harmonics of 500 Hz pulse frequency may play a role as they translate into phase shift. Can be that 11 kHz bandwidth if TL071 is sufficient. I would probably try decompensated OP like LF357 (20 MHz GBW).
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