Looking for ADC for simple 1Msps Propeller oscilloscope

Bean · 2013-11-12 05:08

We have looked around and the best chip seems to be the MAX1377 because it is cheap ~$4 and reads two bipolar inputs at the same time.

http://datasheets.maximintegrated.com/en/ds/MAX1377-MAX1383.pdf

But I haven't been able to find any demo programs or examples of people using it on the web, which kind of scares me off of it.

Also, I'm not sure about reading two SPI inputs at 20MHz and storing the data in the hub.

Has anyone used the MAX1377/1379/1383 before ?

I was just wondering if anyone has any suggestions for an ADC ?

Here is what we need:
1Msps for two channels (could use two chips if needed).
Bipolar inputs (so we don't need a lot of preconditioning circuitry)
At least 8-bits (more is better of course)
Cheap (<$10 for both channels)

Any ideas ?

Thanks,
Bean

Mark_T · 2013-11-12 05:34

The AD9201 looks nice, dual channel differential parallel 10bit upto 20MHz (datasheet seems to imply 1MHz clock will work, but with
parallel interface its easy to read this faster (if you have 10 pins spare).

Rayman · 2013-11-12 05:45

Haven't looked at something like this for a while...
Last time, I was looking for ~100 MHz and got samples of this chip:
AD9283

There is a 50 MHz version that is one channel and $5...

Bean · 2013-11-12 07:32

Mark,
The AD9201 look good, but from reading the datasheet it doesn't allow bipolar inputs.
Rayman, that looks good. I'll have to see if I can afford that many pins.

Thanks guys,

Bean

Rayman · 2013-11-12 08:30

Was just googling and found these chips that will do 1 MSPS with SPI interface (should reduce pin count):
ADS8329 ADS8330

jmg · 2013-11-12 13:44

Bean wrote: »

Looking for ADC for simple 1Msps Propeller oscilloscope

1Msps is fairly light, and many micros offer that now.

If it is for a scope, you might want some logic too, for trigger etc, and so something like this

http://www.digikey.com/product-highlights/en/psoc-4-arm-cortexm0/50179

gives a solution for 1 : $2.53, and just $24 for a Board

OR you might want to jump in speed, and Digikey suggests the MAX1192 is Dual, parallel, 22MSps for $4.38 @ 100
and ~20MSPS seems a better match to Prop peak sample speeds.

Bean wrote: »

Also, I'm not sure about reading two SPI inputs at 20MHz and storing the data in the hub

Surely parallel capture is the best, if the purpose is to build a scope, and there probably two capture paths, with 8 bits of ADC and a 9th trigger/digital bit would seem to fit ?

Or, if you want to be brave, TI has some interesting AFE parts that seem cheap
http://www.ti.com/lsds/ti/data-converters/ccd-cmos-products.page

Digikey has stock of
VSP5621RSLR Texas Instruments IC AFE 16BIT 50MSPS 4CH 48VQFN
4973 100 : $4.3133

The 4 channels at 16 bits is appealing, but the 50Msps is a little creative, as the sustained read rate given is

CMOS 4-bit

MJB · 2013-11-13 14:46

AVR xMegas offer 2 channel 12bit at 2MS/s with max 16 different analog inputs with DMA to memory or to e.g. 8 output pins
modules with 8MB RAM are available cheap.
in scan mode all 16 inputs can be sampled cyclically
- oh - but this is not bipolar ...

jmg · 2013-11-13 16:40

Bean wrote: »

Bipolar inputs (so we don't need a lot of preconditioning circuitry)

I'm not sure what you mean by bipolar ? - but the Data says this

Absolute Voltage Range 0~AVDD Volts

So it can do differential, but it does not go outside the rails.

Reach · 2013-11-13 17:19

Have you checked with Texas Instruments - have them send you a dozen or so free samples. Last time I checked they had several types to chose from but its been a few years.

jmg · 2013-11-13 18:25

I think this takes the prize for ADC speed, in a uC, and it even meets the Cheap (<$10 for both channels) at 1: $8.92

fc(ADC) ADC conversion frequency 12-bit resolution - - 80 MSamples/s

http://www.nxp.com/products/microcontrollers/cortex_m4/LPC4370FET100.html

The 282kRAM, 64K ROM, and various cores and peripherals almost come for free.

The ADC seems to have FIFO and DMA, and can loop via a descriptor state engine + 14b Timer, as it reads.
As Smart ADCs go, this looks quite good

I think the Eval Board is $20, which seems a good price.

Mark_T · 2013-11-14 02:14

jmg wrote: »

I think the Eval Board is $20, which seems a good price.

Well yes, a very good price - is there a link to the eval board?

Bean · 2013-11-14 04:39

jmg wrote: »

I'm not sure what you mean by bipolar ?

I was hoping to get a device that could measure negative voltages (below ground) without the need for a negative reference, but I don't think such a thing exists.

I was getting confused by the output showing negative values, but I can see now that that just means that the negative input is greater than the positive input, NOT that the positive input is below ground level.

I think we will just use a charge pump to get a negative voltage, then use an op amp to buffer the signal, then to the standard propeller delta-sigma type ADC.

Bean

Rayman · 2013-11-14 09:06

I use the Max1270 to read below ground. It's great, but expensive and only does 110ksps.

You can always use a summing circuit to allow you to read voltages below ground with a regular ADC.
You just add a fixed positive voltage to the voltage you want to measure...

Mark_T · 2013-11-14 09:45

Shame there isn't a 1:1 transformer that works at DC!
High bandwidth opamp in differential configuration to mid-rail reference is how
I'd proceed I think.

User Name · 2013-11-14 10:06

Mark_T wrote: »

Well yes, a very good price - is there a link to the eval board?

DigiKey has stock at a $28 price. Just search their site for OM13054.

Also worth noting...a board with a TI version of the ARM Cortex M4 is available for $12.99 right here. It has two 12-channel 12-bit ADCs. This board works great with the free TI IDE (which full disclosure compels me to point out is a big download - but not as big as Quartus!).

Tracy Allen · 2013-11-14 11:37

Like Rayman says, the easy way to read bipolar signals, below ground, is to use a precision resistor divider connected to the (+) reference,

[SIZE=1][FONT=courier new]   Vinput ----/\/\/\----o---/\/\/\-----+Vref
                  R     |       R
                       ADC channel Vadc=(Vref+Vin)/2[/FONT][/SIZE]

That is how the MAX1270 does it internally. Of course the driving impedance has to be low, or accounted for.

Bean · 2013-11-14 11:47

Tracy Allen wrote: »
Like Rayman says, the easy way to read bipolar signals, below ground, is to use a precision resistor divider connected to the (+) reference,
[SIZE=1][FONT=courier new]   Vinput ----/\/\/\----o---/\/\/\-----+Vref
                  R     |       R
                       ADC channel Vadc=Vin/2[/FONT][/SIZE]
That is how the MAX1270 does it internally. Of course the driving impedance has to be low, or accounted for.

That is the problem, with the scope probe disconnected it reads as having a DC voltage on it. So you need an op amp to get a low impedance signal.

Bean

Rayman · 2013-11-14 12:01

Here's a way to do it with opamps:
http://www.ecircuitcenter.com/Circuits/opsum/opsum.htm

Only problem here is that the input impedance would be they show a 10k input impedance. There is no DC level on the tip of the probe though, other end of resistor is kept at ground... Maybe you could do this with the regular 1 Meg input impedance of oscilloscopes, not sure...

jmg · 2013-11-14 12:34

Rayman wrote: »

There is no DC level on the tip of the probe though, other end of resistor is kept at ground... Maybe you could do this with the regular 1 Meg input impedance of oscilloscopes, not sure...

If you want No DC and a classic '1M to ground' Probe load, I think you have to use some negative voltage.(negative current)

It can be quite low current, if it is applied to the bias side only, but then it needs to be well regulated.
With std parts it is probably easiest to do a negative opamp rail and use the opamp PSRR.

I have seen scopes that split DC/HF paths, and use low offset precision parts for DC and HF parts for > 1Khz, but that needs a splitter and re-combine so bumps the parts count.

It is not easy finding well-priced, Scope-performance Opamps.

The Red-Pitaya scope uses a 14b ADC to avoid needing so many scope ranges and they scale to 600mV/1M so a Probe is 6V, which covers most bench needs. Seemed like a good approach, and I like a scope probe that can be a proper voltmeter.

Rayman · 2013-11-14 17:05

Maxim has several ics that can read negative voltage with just a positive supply. But, they all have ~10k input impedance.
As jmg said, I don't see any way around having a negative voltage supply to a buffer amp if you want 1meg input impedance.
But, there are simple charge pumps around that can take +5v and make -5V for you...

Just found a triple opamp with integrated charge pump:
http://www.analog.com/en/all-operational-amplifiers-op-amps/operational-amplifiers-op-amps/ada4858-3/products/product.html

It's for the "video" frequency range, which I think is what you're looking for...

Tracy Allen · 2013-11-14 21:49

+1 that everything is easier with (+)–ground–(-) power supply rails.

Is the 'scope to be portable, battery powered? The ground is then flexible and could be center point between the two batteries, or it could use a single battery and virtual ground IC such as the TLE2425.

Looking for ADC for simple 1Msps Propeller oscilloscope

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