Detection distance for a US nickel and quarter

[Carl-NC]

Why not use an instrumentation amplifier?

Many IA's are not fast enough for PI. Usually you can do as good or better making your own.

Or why not directly sample the fully differential amplifier?

You can do that, if you have a fast enough ADC with enough bits.

If we want to form a power supply of plus or minus five volts, should we use 7905 or 7805 except for the charge pump chip?

That will work. Download a bunch of project schematics to see various solutions.

Our op amp uses a ? 5v power supply, and the microcontroller uses 3.3 or 5v, so how do the op amp and the microcontroller connect to the level?

You may need level shifters, depends on the situation. Again, look at some micro-based projects to see what's been done.

[liudengyuan]

Our op amp uses a ? 5v power supply, and the microcontroller uses 3.3 or 5v, so how do the op amp and the microcontroller connect to the level?

[liudengyuan]

If we want to form a power supply of plus or minus five volts, should we use 7905 or 7805 except for the charge pump chip?

[Altra]

Or why not directly sample the fully differential amplifier?

See jpg in first post

https://www.geotech1.com/forums/showthread.php?17287-field-test-unit-no-001-quot-model-T-quot

[liudengyuan]

Or why not directly sample the fully differential amplifier?

[liudengyuan]

Why not use an instrumentation amplifier?

[Carl-NC]

How do you generate positive and negative power?

You can either use a charge pump to generate it, or use a positive-only supply with a rail splitter to create a midpoint "analog ground." Both methods are widely used in forum projects.

[liudengyuan]

Thanks for the source reference, Eric.
Here is the pdf datasheet for the AD8056 from AD's website:

https://www.analog.com/media/en/technical-documentation/data-sheets/AD8055_8056.pdf

and from Farnell Electronics site:

http://www.farnell.com/datasheets/92690.pdf

And the pertinent part clipped out as jpg images:

How do you generate positive and negative power?

[Skippy]

Thanks for the source reference, Eric.
Here is the pdf datasheet for the AD8056 from AD's website:

https://www.analog.com/media/en/technical-documentation/data-sheets/AD8055_8056.pdf

and from Farnell Electronics site:

http://www.farnell.com/datasheets/92690.pdf

And the pertinent part clipped out as jpg images:

[waltr]

Thanks for posting the source of this circuit.
The AD8056 data sheet gives an explanation.

[Ferric Toes]

Yes, I did make a mistake in the schematic. The boards on which I have used this system all work fine, so the connections there were obviously correct. This idea was based on a 'single ended differential line driver' given as one application in the AD8056 data sheet. I have also seen this arrangement as a phase splitter in audio power amplifiers. A similar result can be made by following the first preamp with an inverter IC. Which is best, I don't know. I assumed the former would be, because of its balanced configuration, which is important in audio.

Eric.

[green]

Left amplifier: offset on top amplifier multiplied by 50, offset on bottom amplifier multiplied by 1. Right amplifier: offset on top or bottom amplifier multiplied by 50. Advantage for either right or left amplifier? Not correct

[green]

Used a sinewave to compare methods reply #67. Should have used a step. Added C1 across 50k left amplifier to adjust response. Both amplifiers take about the same time to settle to correct value(1.3us). I'm guessing the idea is to have the normal and inverted output have the same response so EF cancels. Signal has settled when EF sample is taken for both amplifiers. If target sample is taken to soon both amplifiers have an error. Not seeing an advantage to either amplifier. Not much difference in normal and invert out with either amplifier. What am I missing? Maybe spice simulation doesn't compare well with real circuit?

Left amplifier: offset on top amplifier multiplied by 50, offset on bottom amplifier multiplied by 1. Right amplifier: offset on top or bottom amplifier multiplied by 50. Advantage for either right or left amplifier?

[Skippy]

Each of the amplifiers has the same basic circuit around it, an amplification of +2 ( ie. an inverting amp with gain = -1 ). So dynamically, they will behave the same, any issues like the effect of gain-bandwidth product will affect them equally, likewise they see ( almost) the same load on their output pin, so they will have the same issues regarding gain, phase shifts etc. That means the two complementary outputs will be good inverses .. no different lags, overshoots, settling errors etc.
I think to get the same source resistance to both + and - opamp inputs ( for bias current correction ), the resistors R1, R4, R5, R6 should be 2K0. Then both inputs on both opamps will see 1K0 resistance.
Technically, I think U1 needs a 50K load to 0V ground, so it matches that seen by U2.

[green]

Wondered why 6 resistors instead of 4 resistors. Think I've seen explanation somewhere but don't remember why. Did a spice simulation. At lower frequency both the same, peaks at 1V. Wasn't expecting gain to increase at 1MHz for the four resistor. What are the advantages or disadvantages for each amplifier?