Preamp

[green]

[Most MOSFETS have low Rds_on to work at this level. The peak current would be (6V - 0.8 (series diode)) / Rcoil. Up to 3 A with a 1.7 ohm coil]

I've been using 1v/battery for NiNH and Alkaline batteries(low usable discharge volts). An IRF740 has resistance of about .5 ohms, need more batteries for 3 amps peak current or lower coil resistance. Some have suggested using 1000 to 1500v fets, higher resistance. I've been trying to decide what peak current makes sense.

[Chet]

This is a larger footprint but looks like it could work.

http://www.digikey.com/product-detai...20D-ND/4935608

Have a good day,
Chet

[Teleno]

I've been trying to decide what peak current makes sense.

As much as you can afford makes sense. The longer the current decays, the stronger the signal of the longer taus.

Let's say you have a 300uH coil with a damping resistor of 700 ohms. The max flyback voltage, say 600V

The minimum current you need Imin is the current barely necessary to reach your 600V, which is: Imin = 600V / 450ohm = 1.33 A

Any current above it (Iadd) will decay linearly trhough avalanche (or snubber).

The formula of the linear decay time is:

I(t) = (Iadd + Imin) - V / L * t;

The linear decay period is approx. T = (0.5 * Imin + Iadd) * L / V;

from this, the additional current Iadd will extend the decay times as follows:

Iadd = 1A; T = 1.16 us
Iadd = 2A; T = 1.33 us
Iadd = 4A; T = 2.33 us
Iadd = 8A; T = 4.33 us

Roughly, for any tau > T the signal increases as a function of the current, 2 x more current = 2 x more signal.

For any tau < T the signal does not increase with the current and it is wasted.

[Teleno]

This is a larger footprint but looks like it could work.

http://www.digikey.com/product-detai...20D-ND/4935608

Have a good day,
Chet

Good find!

That thing costs almost $40 in my neck of the woods: https://www.conrad.nl/nl/mosfet-cree...omSuggest=true

Mouser.com offers it for $5 : http://nl.mouser.com/Search/ProductD...41-C2M0280120D

Its low ouput capacitance makes a series diode redundant.

[green]

As much as you can afford makes sense. The longer the current decays, the stronger the signal of the longer taus.

Let's say you have a 300uH coil with a damping resistor of 700 ohms. The max flyback voltage, say 600V

The minimum current you need Imin is the current barely necessary to reach your 600V, which is: Imin = 600V / 450ohm = 1.33 A

Any current above it (Iadd) will decay linearly trhough avalanche (or snubber).

The formula of the linear decay time is:

I(t) = (Iadd + Imin) - V / L * t;

The linear decay period is approx. T = (0.5 * Imin + Iadd) * L / V;

from this, the additional current Iadd will extend the decay times as follows:

Iadd = 1A; T = 1.16 us
Iadd = 2A; T = 1.33 us
Iadd = 4A; T = 2.33 us
Iadd = 8A; T = 4.33 us

Roughly, for any tau > T the signal increases as a function of the current, 2 x more current = 2 x more signal.

For any tau < T the signal does not increase with the current and it is wasted.

Did you pick 3 amps peak in reply #165 because that is all you could afford? If not how much can we afford?

[Teleno]

Did you pick 3 amps peak in reply #165 because that is all you could afford? If not how much can we afford?

Voltage / Resistance, as by law of Nature (Ampere's law).

[Sean_Goddard]

Green try sampling JUST BEFORE the Tx pulse instead of 150us after. Just a thought. Also what about a very LONG ground/ efe sample pulse to integrate as much of the noise as possible?

[Sean_Goddard]

FerChrisake, NO INVERTERS please. If I see a 7660 or the like I'll go mad. Synchronous or not they are just pure noise, and not very stable either. Can I suggest a split rail using an op-amp (around 500mA capability) with a truck load of filtering?

We need supplies so stable they make Cheyenne Mountain look dodgy. I intent to use a programmable load to test the supplies (this included noise injection etc). I think supplies are part of the key to a good PI.

[Teleno]

FerChrisake, NO INVERTERS please. If I see a 7660 or the like I'll go mad. Synchronous or not they are just pure noise, and not very stable either. Can I suggest a split rail using an op-amp (around 500mA capability) with a truck load of filtering?

What battery do you suggest at what voltage?

If around 9V then a -5V regulator LM79L05 would power the MCU. I reference to the + line because the signal is also refereced to that line.

By the way, OpAmps have about 80dB - 100 dB CMRR (common mode rejection ratio). A 100 mV ripple of a converter will result in 1uV at the output, no big deal.

[green]

[Could it be because your GB sample has a higher gain than the main sample to compensate for the loss of target signal?
Therefore higher gain equates to more noise. (Using a longer sample pulse is the same as increasing the gain.)] Reply #162 Qiaozhi

That is some of it, but I don't remember the noise increasing as much as the gain.

[Green try sampling JUST BEFORE the Tx pulse instead of 150us after. Just a thought. Also what about a very LONG ground/ efe sample pulse to integrate as much of the noise as possible?] Reply #172 Sean_Goddard

The GEB sample is about 100usec long. The EF samples are taken just before Tx on.

I'm working on another circuit. My PS noise is to high plus other things. When I get it working I'll post some better measurements.

[Old cart]

Not to be a policeman but we are posting all these comments in the preamp thread. Our original intent was keep this thread clean. While there is lot of good comments here it is hard to keep the thought process going.
I suggest two things:

1 The power supply comments be posted in the power supply section.
2 Once the power supply noise and current requirements are defined we can get back that but there already is considerable discussions on the welcome thread.
Maybe this should be moved to the power supply thread.

Sorry for being a stickler.

[Teleno]

Not to be a policeman but we are posting all these comments in the preamp thread. Our original intent was keep this thread clean. While there is lot of good comments here it is hard to keep the thought process going.
I suggest two things:

1 The power supply comments be posted in the power supply section.
2 Once the power supply noise and current requirements are defined we can get back that but there already is considerable discussions on the welcome thread.
Maybe this should be moved to the power supply thread.

Sorry for being a stickler.

The preamp cannot be independently designed from the power supply, since it works with the raw power at the input and the digital power at the output.

This is my provisional (still partly conceptual) design:

[Old cart]

The preamp cannot be independently designed from the power supply, since it works with the raw power at the input and the digital power at the output.

This is my provisional (still partly conceptual) design:

I agree and noted that before. But specs can be easily passed between threads. Btw you are a very prolific and, I think, good designer. This is a very simple design but a long way from what was originally proposed. I suppose for those that want to stay away from a microprocessor we could do this as two PCBs and just use the output of your preamp to drive a conventional integrator. What are you proposing to use as an ADC for the preamp out? And ten there is the matter of split vs not split supply when it comes to ease of integrating high quality analog.

[Teleno]

I agree and noted that before. But specs can be easily passed between threads. Btw you are a very prolific and, I think, good designer. This is a very simple design but a long way from what was originally proposed. I suppose for those that want to stay away from a microprocessor we could do this as two PCBs and just use the output of your preamp to drive a conventional integrator. What are you proposing to use as an ADC for the preamp out? And ten there is the matter of split vs not split supply when it comes to ease of integrating high quality analog.

The analog part is so designed that it does not depend on a stable power supply. All voltages are referred to one end (the +) and what the other end does is quite unimportant.

The ADC is contained in the MCU and is 12 bit.

If you want to apply this preamp to conventional integrators all the good characteristics will be lost unless you upgrade the integrators to high-speed op-amps ( around 50 Mhz gain-bandwith). Samplig of the Surfmaster has to be redesigned for earlier sampling and faster rise times.

That's why I go MCU, it's just so much simpler. Once the tough analog part is done, programming is the way to go.

[Old cart]

The analog part is so designed that it does not depend on a stable power supply. All voltages are referred to one end (the +) and what the other end does is quite unimportant.

The ADC is contained in the MCU and is 12 bit.

If you want to apply this preamp to conventional integrators all the good characteristics will be lost unless you upgrade the integrators to high-speed op-amps ( around 50 Mhz gain-bandwith). Samplig of the Surfmaster has to be redesigned for earlier sampling and faster rise times.

That's why I go MCU, it's just so much simpler. Once the tough analog part is done, programming is the way to go.

Teleno, I agree. As long as the design can be reproduced AND we can keep the digital noise form the MCU out of the works it is a better and more flexible way to go.
I was mainly concerned about the desires of others:<)
The question then is you going to willing to either provide either preprogrammed MCU's OR to share the code, and instructions on how to program it?