Impedance matching hydrophones and DAQs

[izelkay]

Hello,

I'm confused about the concept of impedance matching in regards with hydrophones and DAQs. If the input impedance to an ADC channel on a DAQ is very high, would I need whatever's being fed into it (hydrophone) to also need a high impedance? Or would it need a low impedance? I've tried researching this but can't find any concrete answers or explanations.

Thanks

 

[davenn]

would I need whatever's being fed into it (hydrophone) to also need a high impedance? Or would it need a low impedance?

so, what is the make and model of your hydrophone ?
is it a high or low impedance source ?
have you got or looked for a datasheet for it ? as it would likely tell you these things

ADC's are generally a voltage driven device, and having a high impedance input is good for 2 reasons

1) higher voltage being developed across the high impedance input

2) It is a good thing for a voltage input, because when the input impedance is high compared to the source impedance then the
voltage level will not drop too much due to the loading effect of the ADC
there are other times for other types of circuits where maximum power transfer is what is required.
Times like this source impedance and load impedance are the same, ie ... matched.Dave

 

[izelkay]

so, what is the make and model of your hydrophone ?
is it a high or low impedance source ?
have you got or looked for a datasheet for it ? as it would likely tell you these things

ADC's are generally a voltage driven device, and having a high impedance input is good for 2 reasons

1) higher voltage being developed across the high impedance input

2) It is a good thing for a voltage input, because when the input impedance is high compared to the source impedance then the
voltage level will not drop too much due to the loading effect of the ADC
there are other times for other types of circuits where maximum power transfer is what is required.
Times like this source impedance and load impedance are the same, ie ... matched.Dave

It's a Reson TC4013 hydrophone, and it's datasheet: http://www.teledyne-reson.com/download/hydrophone_data_sheets__/TC4013.pdf
gives me some information about its impedance at different frequencies. It's much lower than the ADC input of the DAQ (>100 Gohms). So would I need to make these impedances the same?

 

[f95toli]

No, the impedances do not need to be the same unless you are working with RF frequencies (and not always even then). If the signal is a voltage you want the DAQ to have a high impedance (usually the higher the better).
That said, looking at the data sheet I am not at all sure that this hydrophone can be plugged directly into a DAQ. I don't know anything about hydrophones, but assuming they are similar to a condensed microphone you need a microphone amplifier before the DAQ.

 

[izelkay]

No, the impedances do not need to be the same unless you are working with RF frequencies (and not always even then). If the signal is a voltage you want the DAQ to have a high impedance (usually the higher the better).
That said, looking at the data sheet I am not at all sure that this hydrophone can be plugged directly into a DAQ. I don't know anything about hydrophones, but assuming they are similar to a condensed microphone you need a microphone amplifier before the DAQ.

Oh yes, sorry forgot to mention but I'll have the hydrophone go to a preamplifier first, then the DAQ. So would the preamplifier need a high input impedance and a low output impedance? I plan to design/build one

 

[f95toli]

The microphone amplifier you presumably need is not just a simple voltage amplifier (and not all mic amps work well with condenser mics) meaning it is not just a question of input and output impedance. If you want to build one yourself you should find a suitable circuit diagram somewhere.

 

[izelkay]

The microphone amplifier you presumably need is not just a simple voltage amplifier (and not all mic amps work well with condenser mics) meaning it is not just a question of input and output impedance. If you want to build one yourself you should find a suitable circuit diagram somewhere.

Yes I know it won't be simple but I need to start somewhere and I thought impedance requirements was a suitable starting point, but I'm not sure how the input and output impedances should relate. Everywhere I search says something different or isn't clear.

For now I would just like to know how to relate the input/output impedance of a preamplifier if the output goes to a high impedance DAQ and the input to a 4013 hydrophone with frequency dependent impedance. How do I quantify and design for these?

 

[berkeman]

(>100 Gohms).

I'm guessing that is a typo...

 

[izelkay]

I'm guessing that is a typo...

Oh looks like the datasheet is more specific. 100Gohms in parallel with 10pF. Though I'm still not sure what to do with this information. I'm thinking the DAQ input impedance should be really high while the preamp output impedance should be really low since the DAQ is sensing voltage. Similarly, the preamp input impedance should be really high compared to the hydrophone output impedance. Am I on the right track there?

 

[berkeman]

Am I on the right track there?

Yep!

 

[davenn]

I'm thinking the DAQ input impedance should be really high while the preamp output impedance should be really low since the DAQ is sensing voltage. Similarly, the preamp input impedance should be really high compared to the hydrophone output impedance. Am I on the right track there?

yes, that's what I said way back in post #2 which DAQ are you using ?

Dave

 

[izelkay]

yes, that's what I said way back in post #2 which DAQ are you using ?

Dave

Will be interfacing with the NI 6356 next week, but ultimately plan to use an NI PCI-6123.

 

[davenn]

NI 6356

presumably this ?
http://www.ni.com/datasheet/pdf/en/ds-163

interesting unit, haven't played with anything that complex for data acquisition

use something much more straight forward for data logging for my earthquake sensors ( seismometers)
8 channel and 16 bit capability
Dave