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Old 4th April 2004, 01:55 PM   #11
cyr is offline cyr  Sweden
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Join Date: May 2002
Location: Linkoping
Quote:
Originally posted by fr0st


With your input relays, do they selected between 1:1, 10:1 and 100:1?
Most of the TI ADC have 2v p2p differential inputs. In the datasheet they show the use of a THS4502 for the differential driving of the ADC from a single ended signal at a gain of 1. If I use that along with a FET input opamp also at a gain of 1, I would have an input stage capable of a max +- 100v? With a 10x probe that allows for +-1Kv to be measured with a resolution of ~0.1v still (using all 14 bits).
From there I could do software scaling in order to fit the waveform to the screen.
I assume thats what you've done for your original DSO.
Do you have the schematic of your final analog stage?

Thanks
Matt
That's right, 1:1, 1:10 or 1:100 and then 5x or 1x gain, fixed 2x gain and an additional 2x selectable gain by lowering the ADC reference voltage.

I only have the very messy eagle schematic I made when laying out the board for this second prototype. I will draw a clearer, simplified schematic and put it up on my site soon.

Also, it's not actually my final design. I already have some changes planned for the next (hopefully final) version and I will probably make more after some more testing of this version.

Meanwhile, here's a photo of the actual hardware:
Click the image to open in full size.

Note that this 8x10cm board is only the sampling module. There will be up to four of these connected to a "motherboard" with the FPGA and memory.

A closeup:
Click the image to open in full size.
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Old 5th April 2004, 09:07 AM   #12
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Quote:
If you look at the Linear Tech application notes for the LTC1446 DAC there is a schematic for autoscaling which you might want to implement.

with respect to offset, there are precision op-amps with auto-offset features (from TI)

if the device is going to have any meaningful bandwidth you might want to consider using op-amps with "disable" function rather than inserting switches in the signal path. TI will send you samples.

When you begin talking of inputs in the 1kV range you are entering into a different strata of measurement and control. Note that some resistors become non-linear over a few hundred volts, note also that with just a little current you can fry a resistor that isn't rated for HV.
I had a look through most of TI's range of op amps and only a few have a disable function. I'm unsure of how it would be implemented though.
If you disable the opamp the output goes into a high impedence state, so the only way switching gains would be to cascade to opamps and have one on whilst the other is off. Swithcing between them will cause the switch in gain, the only thing is, if you cascade them the external components would interfere (well I assume so) because parts of the feedback would be tried to ground which i assume would alter something (frequency respone, output swing, feedback?).
The THS7002 would be an easy way out by sacrificing bandwidth (-3db at 70Mhz). Although its unlikely I'd be measuring a signal greater than 70Mhz it'd still be nice to have the extra bandwidth, but it is an option.
With the auto Offset opamps, do you know which one you are refering to? I had a look at the precision section quickly but none I looked at say the adjusted to drift, but only had very little of it.
Also in relation to measuring a 1kV signal, The input stage should never see the full 1kV. Using a 10x probe, the kV signal is reduced down to 100v and fed to the input stage.

Quote:
That's right, 1:1, 1:10 or 1:100 and then 5x or 1x gain, fixed 2x gain and an additional 2x selectable gain by lowering the ADC reference voltage.

I only have the very messy eagle schematic I made when laying out the board for this second prototype. I will draw a clearer, simplified schematic and put it up on my site soon.

Also, it's not actually my final design. I already have some changes planned for the next (hopefully final) version and I will probably make more after some more testing of this version.
Arn't the ratios 10:1, 100:1 instead of 1:10, 1:100? Otherwise you owuld be amplifying the signal

Thanks
Matt
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Old 5th April 2004, 12:35 PM   #13
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Quote:
Originally posted by fr0st

I had a look through most of TI's range of op amps and only a few have a disable function. I'm unsure of how it would be implemented though.
how about this:
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Old 7th May 2004, 05:47 PM   #14
gooftar is offline gooftar  France
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Smile Another dso project

Hi there,

I am currently doing a prototype dso using THS7530 variable gain amplifier from TI for input stage.
the first DAC7571 sets the gain and the second makes the trig level.

The adc is a ADS831 (8bit - 80 Msps)

The sample clock is generated by an econoscillator from maxim (DS1085).

See the attached schematic.



I am currently evaluating the analog part.

What do you think of this design ?
Click the image to open in full size.

Sorry for the large pic.


Bye
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