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Fast Prec I to V - problem with noise

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maziar

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Fast Prec I to V

i try convert multi range(10ma to 10na in 8 range) I to v converter with hi prec (12bit) in high speed (higer 500khz)

i have problem with noise .

how resolve it ?
have av. sngle chip I to V with wide range to na ?
 

gelly

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noise

hi

normally 12 bit is at the edge for a scale of 1000 (for na values).
-did u decouple the i/v output voltage with small ceramic condenser ?
-are your measuring wires not too long ?
-try to 'transport' that signal with a thin coax cable (schield grounded 1 s.)
-decouple your i/v converter pwr with 100n to decrease noise on the VDD.
-are the chosen values for resistors as small as possible ?
(the bigger the resistor, the bigger the noise it can produce)

hope this helps.

gelly.
 

Jensen

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Re: noise

Hi Maziar,

I don't sure what kinds of noise in the problem as your mention above.

As my knowledge, in this case, resistors values, longness of decouple capacitor's lead to i/v converter supply pin, what kinds of decouple capacitor, i/v converter chip, and so on are very importance.

My suggestion is use the SMD package component, use small ceramic capacitor & chosen low values for resistors as gelly's speech.
Reduce wire longness of decouple cap. to i/v converter chip's supply pin.

Finally, i suggest u to use the AD846AR for your application. I thing the AD846 is the best choice for you.

Good luck
 

maziar

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THX

i use AD845 and simple diagram of I/V Converter my connection no long and is shield.
with 10uTant + 100n ceramic bypass with good +-12V Reg supply


but in high speed current wave graph noise is very high.

have any proffissonal Fast(500khz)wide range prec SCH for low current range I/V converter ?
 

Jensen

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Hi

what's reg. supply be used in your application? Switching power?

If so, try to use a linear reg. to replace it. In this case pls forget the switching power.

By the way, what is the current source of this i/v circuit? A high speed dac? Does it need a lpf to filter the sampling noise?
 

maziar

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Jensen said:
Hi

what's reg. supply be used in your application? Switching power?

If so, try to use a linear reg. to replace it. In this case pls forget the switching power.

By the way, what is the current source of this i/v circuit? A high speed dac? Does it need a lpf to filter the sampling noise?
1. LM317 REG with .3mvpp noise !(no any noise in Power supply)
2. input of instrument device and dont forget is High speed wave (100khz sampling rate)
 

7rots51

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Do not forget the themocouple effects in junctions and connectors that changes with room temperature changes.

Use good ground plane for your pcb,and use seperate analog and digital grounds ,...
 

maziar

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RTOS51 said:
Do not forget the themocouple effects in junctions and connectors that changes with room temperature changes.

Use good ground plane for your pcb,and use seperate analog and digital grounds ,...

1. termocouple in offsset no noise.i have noise !
2. i use full signal isolated ground
 

Jensen

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100khz sampling rate, why you need 500khz bandwide for i/v converter.
try to reduce bandwide to rejection noise.
 

maziar

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Jensen said:
100khz sampling rate, why you need 500khz bandwide for i/v converter.
try to reduce bandwide to rejection noise.

i think forget: 500khz bandwidth write for sine wave responce for 30% rej.

then if you need pulse responce of settling time this number (5x) is true
 

Jensen

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maziar said:
i think forget: 500khz bandwidth write for sine wave responce for 30% rej.

then if you need pulse responce of settling time this number (5x) is true

1. LM317 REG with .3mvpp noise !(no any noise in Power supply)
2. input of instrument device and dont forget is High speed wave (100khz sampling rate)
Yep, that's ture. If you need pulse responce of settling time that you need(5x) bandwidth.
But as your speech, the sampling rate is 100khz. According to the sampling law you may need a anti-ailising filter to reduce the bandwide.
Again, way you need 500khz bandwide.
 

shanren

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hi,maziar:
i think you must use a low noise OP to do the job,and use low noise component ,such as MKP polymer capacitors.w*w.linear.com and w*w.maxim-ic.com and w*w.anlog.com or w*w.ti.com have high prec OPs and many app notes.
and you must use a good regulator ,LM317 can not reduce hf noise.you can use LDO ,most can do well than LM317.add a LC or RC filter in power supply will be helpful to reduce power noise.and use a big capacitor is good choise,such as a 470u tan cap,and the sanyo OS-CAP is very good for your design.
good luck!
 

maziar

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my power is very good and have not any noise !

but i problem i to v converter in high speed and high prec
 

maziar

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shanren said:
do you mean you need a high precision op and a faster AD?if so ,you can get ops and AD converters from:
h**p://www.ti.com
h**p://www.linear.com
h**p://www.anolog.com
they also have many app notes.
hope this can help u.

no ineed single chip (analog) for multi Range Fast I/V converting this chip muse better 12 resolution
 

shanren

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ad7866 is a 12bit 1msps chip from analog,this maybe what you need.and AD9240is a 14bit 10msps ad converter chip.a sigal op can do a i-v job,and you can change the gain of op to get the range yu need.try:
http://www.analog.com/bulletins/adc3
to get what you need.
good luck.
 

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