Data acquisition System Design

hi,

I want to design a single channel Data acquisition system based on the following link. See the link

***sine.ni.com/nips/cds/view/p/lang/en/nid/1865

What type of front end circuitry is used in this design?

I want to measure low voltage level 200mV PP.

Hi,

I see a broken link and a huge manual. Both are not useful informations.

Please write down your tecnical soecifications, mostly as units with values..
Then ask an elaborated question.

Klaus

Thanks for reply.

I want to measure micro-volt level signal and digitize it using 12bit ADC , in a 50 Ohm system. The bandwidth is 20KHz. I want to know the following

1) input stage circuit type
2) Total number of stages required
3) Filter if required
4) Input offset / input bias / Noise issues
5) Op-amp Selection

The system will acquire signals from 50 ohm, 20-50 meter coaxial cable.

Hi,

It's difficult to answer.
We know:
* Input voltage range: +/-100mV (not sure)
* 0 ... 20kHz analog input frequency range
* 50 Ohms (rather unusual for 20kHz bandwidth)

Not sure if you are interested in DC accuracy (not important for audio, but often important for sensor signals)
Not sure if you are interested in very low THD (important for audio, but rarely important for sensor signals)
Not sure if your input signal is already bandwidth limited
Not sure if you want to go for the lowest possible sampling rate, or maybe you want a lot of sample points even on a 20kHz signal
Not sure what ADC you use and what specifications it has

1) depends on ADC input range and expected headroom
2) what "stages" do you refer to?
3) filter is required if the input signal contains: frequency_components >= sampling_rate/2, and if alias frequencies harm your further signal processing
4) we don't know your requirements
5) we don't know your requirements

Klaus

Thanks for reply

Not sure if you are interested in DC accuracy (not important for audio, but often important for sensor signals)

-> the input signal is always AC.
Not sure if you are interested in very low THD (important for audio, but rarely important for sensor signals)
-> Not interested in THD
Not sure if your input signal is already bandwidth limited
-> Signal is not bandwidth limited.
Not sure if you want to go for the lowest possible sampling rate, or maybe you want a lot of sample points even on a 20kHz signal
-> sampling rate in 200KHz fixed
Not sure what ADC you use and what specifications it has
ADC is AD574

1) depends on ADC input range and expected headroom
2) what "stages" do you refer to?
At input stage should i use single ended or differential type op amp amplifier ?
3) filter is required if the input signal contains: frequency_components >= sampling_rate/2, and if alias frequencies harm your further signal processing
4) we don't know your requirements
DO i need ultra low noise , low drift type op amp.
5) we don't know your requirements
How to chose op amp , general purpose , FET based Input etc

Hi,

ADC is AD574

Click to expand...

This is an extremely expensive, outdated ADC without sample and hold circuit. It needs three power supplies...
Your choice couldn´t be much worse.

May I ask why you don´t choose a modern, cheaper ADC with much better features and better performance?
--> every distributor and every ADC manufacturer has interactive selection guides to choose a more suitable ADC.

*****

-> Signal is not bandwidth limited.
Not sure if you want to go for the lowest possible sampling rate, or maybe you want a lot of sample points even on a 20kHz signal
-> sampling rate in 200KHz fixed

Click to expand...

According Nyquist you need to suppress all frequencies at 100kHz and above to a very low level. This is called anti-aliasing filter.
In your case I recommend at least a 4th order filter with cutoff frequency of about 25kHz.
It will attenuate 20kHz only a little, and it will suppress alias frequencies to less than 1%

*****

Not sure if you are interested in DC accuracy (not important for audio, but often important for sensor signals)
-> the input signal is always AC.

Click to expand...

Then I recommend an AC coupled input stage.

*****

2) what "stages" do you refer to?
At input stage should i use single ended or differential type op amp amplifier ?

Click to expand...

You talk about a coax cable --> this means single ended signal input to your amplifier
You talk about AD674 --> this means the amplifier output needs to be single ended, too.

****
Please give more informations. Please all in one post. Otherwise helping becomes inefficient.

You are not interested in DC and you are not interested in low THD, so what are you interested in?


Klaus

"You are not interested in DC and you are not interested in low THD, so what are you interested in?"

I dont understand it clearly.

Hi,

"You are not interested in DC and you are not interested in low THD, so what are you interested in?"

I dont understand it clearly.

Click to expand...

You want to measure a signal.
What do you do with the measured signal?
Usally one calculates something and shows the calculated value at a display or stores the value somehow...

So what value you want to calculate? What is your measurement used for?


Klaus

thanks for reply.

When the signal increases from a fixed threshold then it will cause an alarm.

Hi,

You give rather vague informations, slowly, piece by piece.
Please understand that I want to limit my assistance to two additional posts in this thread.
So please give detailed informations and elaborate your questions.

***************

When the signal increases from a fixed threshold then it will cause an alarm.

Click to expand...

What do you mean with "signal"?
* It can be a peak voltage
* it can be an average DC voltage (which makes no sense, because you said the input is AC only)
* it can be AC RMS
* it can be frequency
* it can be a lot of other parameters

We don´t know...

Klaus

Thanks for reply.

Basically it is fluctuation/glitch type signal (when some other device turns on/off).

- - - Updated - - -

not a proper frequency , its like impulse etc

For simplicity let us assume the signal is 1kHz sine wave with 50uV PP. It will appear for few mili-seconds .

I have LM7171, THS4001 , LF358 and LT1357 type op amps . Can i use to make low noise amplifier (assuming bandwidth 25KHz)

Hi,

You want us to read the datasheets for you and compare the noise specifications?

hint: open them in a PDF reader and do a search for "noise". It least this is how I´d do it.

Klaus

Thanks for reply.

I know noise parameters given in datasheet. But the following things make me confused.

For a standard inverting amplifier with 100 Gain, how do you chose the device? Suppose an amplifier built around LM358 to amplify 10 KHz sine wave, with input resistance of 1 K and feed back resistor of 100K. If i use a op amp with FET input device (LF353) or a high speed op- amp like LM7171, the same resistance values will work?

Hi,

The most important thing you need to know: The lower the noise specification in the datasheet, the better.

The next: The lower the resistor values, the less noise.. but the the current must be well within the OPAMP specification.

In your case: 1k and 100k are good values for standard circuits.
But you may go down to 50 ohms and 5kOhms. Then you don´t need the extra 50 Ohms termination resistor usually necessary for the 50Ohms coax cable.
The 5kOhms should cause max 1mA at 5V. The OPAMP needs to be able to drive this 1mA easily. Check datasheets.

What we don´t know: Do you need the 50Ohms termination, and is the signal source (at the other end of the cable) able to drive the 50 Ohms?
(I know it´s annoying that I ask for informations repeatedly. I will suppress it in future)

What makes me wonder: In post #1 you talked about 200mVpp, now you talk about 50uVpp --> you changed specifiactions by a factor of 4000!! This is huge.
Thus many recommendations made for post#1 are useless now.
*****

Back to OPAMP:
Within your last posts you talk about: 1kHz, 25kHz and 10kHz....(at least only a factor of 25)
Let´s take 25kHz and a gain of 100, this means your OPAMP needs a GBW of at least 25kHz x 100 = 2500kHz = 2.5MHz.
(Use a headroom- factor of at least 5 to the true max frequency_of_interest)


Klaus

LM358 won't be able to amplify 10 kHz signal by 100. Review datasheet, start basic calculations.

Selection of resistors is driven by several consideration, e.g. noise, OP input current, source impedance, parasitic circuit capacitances, bandwidth.

I am very thankful for you replies to my odd question.

I don't have experience in OP-Amps practical circuits.

------------------------------------------------------------------------

i need 50 ohm termination and source can drive 50 ohm cable.

------------------------------------------------------------------------

"What makes me wonder: In post #1 you talked about 200mVpp, now you talk about 50uVpp --> you changed specifiactions by a factor of 4000!! This is huge."

Whatever the signal level is the amplifier should amplify the signal, am i correct?

Hi,

Whatever the signal level is the amplifier should amplify the signal, am i correct?

Click to expand...

Generally: Yes
In detail: No.
* There are lower limits. Mainly the noise level will determine the lowest useful signal level.
* and there are upper limits: Output voltage limit, rise and fall rate limits, Gain*bandwidth limits.

Klaus

Thanks for reply.

[https]://gasstationwithoutpumps.wordpress.com/2013/07/07/2-op-amp-instrumentation-amp/

i am trying to simulate the circuit using LM318 in TINA simulator. It works well. But when i change the opamp like LF353 or any other with JFET input type device , i don't get output. There is always constant value at output.

Hi,

it´s your decision whether you show us your tina simulation or not....

Klaus