Low Noise Front End for P.I.

[Unregistered]

AD797

Don't know how well it will work in a PI frontend but Minelab uses them in the Tornado series of coils for the Sov Elite and GT. So they do work well for VLF Front ends.

JMHO

Unregistered

[Ferric Toes]

Ferric Toes,
I apreciate very much your posts. Would you be so kind as to explain the scope pics to me? With the scope at 2uS ?,div. What is that spike of less than halve a division on the second pix?
On the first pix it looks to me that the fly back is overdamped and oscillates. If this is caused by the change of the opamp, it suggests to me that there is additional input capacitance present.
Tinkerer

Hi Tinkerer,

The trace starts at the left, on the termination of the TX pulse. Ideally it would swing into negative saturation (inverting opamp) for the duration of the back emf pulse, and then climb swiftly and smoothly back to a zero volt base line. The first trace is somewhat under damped, giving the overshoot that you see. A bit of overshoot is of no great significance, as long as sampling is done after it is over. You can see that the bandwidth/slew rate is limiting the speed of the response by the slope on the waveform edges.

The second trace is using the dual opamp arrangement which has the advantages of faster response, because the first opamp is run at much lower gain, and also the gain recovers very rapidly as explained before. The blips on the trace are the points at which the sample gates are operating. This is caused by a small capacitive breakthough of the sample pulse edges.

The was an error in my previous post, where I said that the scope TB was 2uS/div, in fact it was 5uS/div. So the gain reduction is also happening at times more pedestrian.

Eric.

[Stefan]

An Interesting application note for a PI Front end

I've attached a zip file for an application note I picked up quite some time ago. It converts an INA103 Instrumentation amplifier configured as an op amp into a high gain amplifier having a gain bandwith of 600 Mhz. It's an interesting circuit and hopefully may be of use to someone.


Stefan.

[JC1]

EE

5us !!!, Oh gee, was looking forward to 10 us samples.

[Ferric Toes]

5us !!!, Oh gee, was looking forward to 10 us samples.

Presume you mean 10uS delay? If so, no problem. Even 5uS delay is standard for some industrial uses. The coil and transmitter needs to be optimised in these cases though, to reduce capacitance.

Eric.

[JC1]

EE

Hi Eric,

Yes I meant delay. Have you been able to get 10 us delay with about 2 amps of coil current?

[_Raz_]

Have you been able to get 10 us delay with about 2 amps of coil current?

I can't speak for Eric, but I am using approx 2A coil current and the best I have been able to achieve is around 12uS sample delay on a mono and around 9uS on a DD. This is using Minelab/coiltek commercial coils.

[_Raz_]

I got a AD797 today and plugged it into my board. No go, which is puzzling. This one stays in positive saturation too long.

Damn, it looked so good in the datasheet too. Thanks for posting the results, it will save me from ordering some, only to be disapointed.

Has the circuit in the above link been built, and worked successfully?

I have no idea. Perhaps the project only got to the schematic/concept stage. There hasn't been much posted on the project since the original schematic post.

Looks like I will stick with the 5534/8055 combo, which with the lower value resistors seems to be as good as it gets at the moment. As JC1 suggested, there is probably a more exotic way of doing things to get lower noise, but I don't have the time at the moment, to start a brand new approach.

I think I'll stay with the 5534 also. The alternatives seem to be a lot of work, with very little return. The law of diminishing returns applies here I think.

Mark

[Cossaro]

Damn, it looked so good in the datasheet too. Thanks for posting the results, it will save me from ordering some, only to be disapointed.

I have no idea. Perhaps the project only got to the schematic/concept stage. There hasn't been much posted on the project since the original schematic post.

Mark

Hello!
This one work, only some little additions on the first "driver" block.
The preamp AD797 + OP37 was untouched. I use a series of two 1n4148 to the input stage and the input don't go on saturation with 60-120 microsec pulse on 300 microH coil.

The digital schematic has a little change in the pull-up on AtMega PORTC.
I'm still working in the reduction of power noise and cpu noise!!!

This is the main limit on this project, the CPU injects a lot of noise in the -5 rail, better a solution with a double 7905 (one dedicated to the digital section).

Still here to post some news.
(have no time to develop my hobby!!!)

[Muntari]

opamp and noise

Hi ,

I am currently using the LMH6624 from Natsemi, however I am using it with a gain of 10 followed by a VCA8613 with programable gain from 10 to 100. Using 8 channels of this device with the Natsemi Opamp as the front end to each channel it is a nicely matched combination for my design. Downside for some PI folk might be that it only comes in SMD and the output voltage swing is limited also output impedence is quite high. It is definitely not a direct replacement for the NE5534 however it is worth looking at along with some others from NS,TI and AD albeit with required PI circuit changes to suit.
I prefer to use more than 1 gain stage with high gain circuits, I just find them much more stable than single opamp high gain stage layouts.







Cheers

Brian K

[Muntari]

opamp and noise

Hi ,

I am currently using the LMH6624 from Natsemi, however I am using it with a gain of 10 followed by a VCA8613 with programable gain from 10 to 100. Using 8 channels of this device with the Natsemi Opamp as the front end to each channel it is a nicely matched combination for my design. Downside for some PI folk might be that it only comes in SMD and the output voltage swing is limited. Slew rate is quite high. It is definitely not a direct replacement for the NE5534 however it is worth looking at along with some others from NS,TI and AD albeit with required PI circuit changes to suit.
I prefer to use more than 1 gain stage with high gain circuits, I just find them much more stable than single opamp high gain stage layouts.







Cheers

Brian K

[Muntari]

oops

Sorry,
While editing the post I pressed submit...it's getting late..!
disregard first post......second one is correct.

If enough people are interested I will draw up and post a cross reference list on low noise opamps, much the same as the Mosfet one I posted sometime ago.


cheers
Brian K

[Ferric Toes]

Hi ,

I am currently using the LMH6624 from Natsemi, however I am using it with a gain of 10 followed by a VCA8613 with programable gain from 10 to 100. Using 8 channels of this device with the Natsemi Opamp as the front end to each channel it is a nicely matched combination for my design. Downside for some PI folk might be that it only comes in SMD and the output voltage swing is limited. Slew rate is quite high. It is definitely not a direct replacement for the NE5534 however it is worth looking at along with some others from NS,TI and AD albeit with required PI circuit changes to suit.
I prefer to use more than 1 gain stage with high gain circuits, I just find them much more stable than single opamp high gain stage layouts.

Hi Brian,

Thanks for the info that you are using a LMH6624. Reg suggested that one to me as a possibility, but the price over here put me off. I had already spent quite a few £ on opamps that didn't work for this application. I had some free samples of LT1007 and LT1037 this morning, but they do not work either. This is as a x10 stage in front of a x50 AD8055. I will contact National Semi here and see if I can get a freebie.

Eric.

[Muntari]

Hi Brian,

Thanks for the info that you are using a LMH6624. Reg suggested that one to me as a possibility, but the price over here put me off. I had already spent quite a few £ on opamps that didn't work for this application. I had some free samples of LT1007 and LT1037 this morning, but they do not work either. This is as a x10 stage in front of a x50 AD8055. I will contact National Semi here and see if I can get a freebie.

Eric.

Hi Eric,
I have a couple left from my samples and can drop them in the post to you if you like , not a problem. Email me with your PA if you can't get local and I will send.

Cheers
Brian

[ElectroNovice]

Would this op amp be of any use?

Would the AD 8099 be of any use?
see:http://www.analog.com/UploadedFiles/...34AD8099_b.pdf
ElectroNovice