current amplifier or current to voltage op-amp

Hi,

I want to measure very weak piezoelectric signal from a quartz crystal dete=
ctor that mechanically vibrate.

Although I can simply measure the output using a preamp, such as SRS 560, i=
t seems I will need a "trans impedance amplifier" to convert the current si=
gnal to voltage signal, in order to provide a virtual ground and avoid load=
ing the detector, if my understanding is right.

There are commercial current amplifiers such as SRS 570:
http://www.thinksrs.com/products/SR570.htm

but I can also use the transimpedance op-amp circuit with OPA 2355.

Or I can just use an ordinary op-amp to connect the detector to its input a=
nd use a feedback resistor, then connect to the voltage preamp.

which of these options are the correct one, without losing the sensitivity?

Thank you!

Xunchen

On Tue, 11 Feb 2014 15:04:30 -0800 (PST), xunchen@ualberta.ca wrote:

>Hi,
>
>I want to measure very weak piezoelectric signal from a quartz crystal detector that mechanically vibrate.
>
>Although I can simply measure the output using a preamp, such as SRS 560, it seems I will need a "trans impedance amplifier" to convert the current signal to voltage signal, in order to provide a virtual ground and avoid loading the detector, if my understanding is right.
>
>There are commercial current amplifiers such as SRS 570:
>http://www.thinksrs.com/products/SR570.htm
>
>but I can also use the transimpedance op-amp circuit with OPA 2355.
>
>Or I can just use an ordinary op-amp to connect the detector to its input and use a feedback resistor, then connect to the voltage preamp.
>
>which of these options are the correct one, without losing the sensitivity?
>
>Thank you!
>
>Xunchen


Google:  charge amplifier



-- 

John Larkin         Highland Technology, Inc

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

On Wednesday, 12 February 2014 10:04:30 UTC+11, xun...@ualberta.ca  wrote:
> Hi,
>=20
> I want to measure very weak piezoelectric signal from a quartz crystal de=
tector that mechanically vibrates.=20
>=20
> Although I can simply measure the output using a preamp, such as SRS 560,=
 it seems I will need a "trans impedance amplifier" to convert the current =
signal to voltage signal, in order to provide a virtual ground and avoid lo=
ading the detector, if my understanding is right.
> =20
> There are commercial current amplifiers such as SRS 570:
>=20
> http://www.thinksrs.com/products/SR570.htm
> =20
> but I can also use the transimpedance op-amp circuit with OPA 2355.
> =20
> Or I can just use an ordinary op-amp to connect the detector to its input=
 and use a feedback resistor, then connect to the voltage preamp.
>=20
> which of these options are the correct one, without losing the sensitivit=
y?

That probably depends on your quartz crystal detector.

Its impedance is going to include an appreciable capacitance to ground - yo=
ur op amp circuit will have to be designed to cope with that capacitance. I=
f you use just a feed-back resistor to the virtual ground, the parallel cap=
acitance of the resistor (typically 0.3pF but some surface mount parts have=
 less) may not be enough to keep the circuit stable.

The Burr-Brown data sheet for the rather faster OPA656

http://www.ti.com/lit/ds/symlink/opa656.pdf

includes a useful discussion of it's use as a transimpedance amplifier.

--=20
Bill Sloman, Sydney

"Bill Sloman"

That probably depends on your quartz crystal detector.


** The only references I can find to "quartz crystal detector" all involve 
frequency measurement of a crystal oscillator.

So WTF is the dopey OP on about  ??

Surely it must be a  " piezo electric " sensor.

Be an idea to clear up the ambiguities in his post first - eh ?

Though I know that goes against usenet's " lets all play a guessing game " 
culture.




....   Phil

Thanks.
I think the circuit should be something similar to the front end circuit de=
sign such as the one by Philip Hobbs
http://www.osa-opn.org/abstract.cfm?URI=3Dopn-12-4-44

but I am just intimidated to build the circuit myself.
If the commercial one such as SRS 570 will do the job, I tent to buy it.
In my breadboard test with a ordinary op-amp tl082cp, the noise level is ve=
ry high.=20


On Tuesday, February 11, 2014 8:22:49 PM UTC-7, Bill Sloman wrote:
> On Wednesday, 12 February 2014 10:04:30 UTC+11, xun...@ualberta.ca  wrote=
:
>=20
> > Hi,
>=20
> >=20
>=20
> > I want to measure very weak piezoelectric signal from a quartz crystal =
detector that mechanically vibrates.=20
>=20
> >=20
>=20
> > Although I can simply measure the output using a preamp, such as SRS 56=
0, it seems I will need a "trans impedance amplifier" to convert the curren=
t signal to voltage signal, in order to provide a virtual ground and avoid =
loading the detector, if my understanding is right.
>=20
> > =20
>=20
> > There are commercial current amplifiers such as SRS 570:
>=20
> >=20
>=20
> > http://www.thinksrs.com/products/SR570.htm
>=20
> > =20
>=20
> > but I can also use the transimpedance op-amp circuit with OPA 2355.
>=20
> > =20
>=20
> > Or I can just use an ordinary op-amp to connect the detector to its inp=
ut and use a feedback resistor, then connect to the voltage preamp.
>=20
> >=20
>=20
> > which of these options are the correct one, without losing the sensitiv=
ity?
>=20
>=20
>=20
> That probably depends on your quartz crystal detector.
>=20
>=20
>=20
> Its impedance is going to include an appreciable capacitance to ground - =
your op amp circuit will have to be designed to cope with that capacitance.=
 If you use just a feed-back resistor to the virtual ground, the parallel c=
apacitance of the resistor (typically 0.3pF but some surface mount parts ha=
ve less) may not be enough to keep the circuit stable.
>=20
>=20
>=20
> The Burr-Brown data sheet for the rather faster OPA656
>=20
>=20
>=20
> http://www.ti.com/lit/ds/symlink/opa656.pdf
>=20
>=20
>=20
> includes a useful discussion of it's use as a transimpedance amplifier.
>=20
>=20
>=20
> --=20
>=20
> Bill Sloman, Sydney



On Tuesday, February 11, 2014 8:22:49 PM UTC-7, Bill Sloman wrote:
> On Wednesday, 12 February 2014 10:04:30 UTC+11, xun...@ualberta.ca  wrote=
:
>=20
> > Hi,
>=20
> >=20
>=20
> > I want to measure very weak piezoelectric signal from a quartz crystal =
detector that mechanically vibrates.=20
>=20
> >=20
>=20
> > Although I can simply measure the output using a preamp, such as SRS 56=
0, it seems I will need a "trans impedance amplifier" to convert the curren=
t signal to voltage signal, in order to provide a virtual ground and avoid =
loading the detector, if my understanding is right.
>=20
> > =20
>=20
> > There are commercial current amplifiers such as SRS 570:
>=20
> >=20
>=20
> > http://www.thinksrs.com/products/SR570.htm
>=20
> > =20
>=20
> > but I can also use the transimpedance op-amp circuit with OPA 2355.
>=20
> > =20
>=20
> > Or I can just use an ordinary op-amp to connect the detector to its inp=
ut and use a feedback resistor, then connect to the voltage preamp.
>=20
> >=20
>=20
> > which of these options are the correct one, without losing the sensitiv=
ity?
>=20
>=20
>=20
> That probably depends on your quartz crystal detector.
>=20
>=20
>=20
> Its impedance is going to include an appreciable capacitance to ground - =
your op amp circuit will have to be designed to cope with that capacitance.=
 If you use just a feed-back resistor to the virtual ground, the parallel c=
apacitance of the resistor (typically 0.3pF but some surface mount parts ha=
ve less) may not be enough to keep the circuit stable.
>=20
>=20
>=20
> The Burr-Brown data sheet for the rather faster OPA656
>=20
>=20
>=20
> http://www.ti.com/lit/ds/symlink/opa656.pdf
>=20
>=20
>=20
> includes a useful discussion of it's use as a transimpedance amplifier.
>=20
>=20
>=20
> --=20
>=20
> Bill Sloman, Sydney

I am using the quartz tuning fork as a very sensitive force detector to pick up sound wave at its resonance frequency. 
So I need to measure the very weak piezoelectric signal. 
I read some paper saying a custom made transimpedance amplifier is needed to do the I-V conversion and then a voltage preamp can be used.
So I am thinking if a commercial current preamp will also work.

On Tuesday, February 11, 2014 8:29:42 PM UTC-7, Phil Allison wrote:
> "Bill Sloman"
> 
> 
> 
> That probably depends on your quartz crystal detector.
> 
> 
> 
> 
> 
> ** The only references I can find to "quartz crystal detector" all involve 
> 
> frequency measurement of a crystal oscillator.
> 
> 
> 
> So WTF is the dopey OP on about  ??
> 
> 
> 
> Surely it must be a  " piezo electric " sensor.
> 
> 
> 
> Be an idea to clear up the ambiguities in his post first - eh ?
> 
> 
> 
> Though I know that goes against usenet's " lets all play a guessing game " 
> 
> culture.
> 
> 
> 
> 
> 
> 
> 
> 
> 
> ....   Phil

On 12/02/14 16:07, xunchen@ualberta.ca wrote:
> I am using the quartz tuning fork as a very sensitive force detector to pick up sound wave at its resonance frequency.
> So I need to measure the very weak piezoelectric signal.

Maybe that's not as hard as you think it is.
The tuning fork has a very high Q, which will
give you massive voltage (and impedance)
amplification. I suspect you need a FET-input
buffer amp and possibly not very much voltage
gain.

"Clifford Heath"
> On 12/02/14 16:07, xunchen@ualberta.ca wrote:
>
>> I am using the quartz tuning fork as a very sensitive force detector to 
>> pick up sound wave at its resonance frequency.
>> So I need to measure the very weak piezoelectric signal.
>
> Maybe that's not as hard as you think it is.
> The tuning fork has a very high Q, which will
> give you massive voltage (and impedance)
> amplification.

** Hang on a mo.

IF  the OP is using a clock crystal ( 32.768 Hz ) then resonance is at 
impedance minimum.

So his notion that current is key is right.

Op-amps with very good noise figures at impedances between a few hundred and 
a few thousand ohms are easy to get.

Doubt he needs to go for fancy lab pre-amp units.



...  Phil

On Wednesday, 12 February 2014 16:07:10 UTC+11, Xunchen Liu  wrote:

<snip>

Phil said

> > Surely it must be a  " piezo electric " sensor.
> > =20
> > Be an idea to clear up the ambiguities in his post first - eh ?=20
> > =20
> > Though I know that goes against usenet's " lets all play a guessing gam=
e " =20
> > culture.

I did ask, perhaps more diplomatically ad covertly than you would have done=
.

And he has given us some more information - though rather less than enough.

> I am using the quartz tuning fork as a very sensitive force detector to p=
ick up sound wave at its resonance frequency.=20
>=20
> So I need to measure the very weak piezoelectric signal.=20
>=20
> I read some paper saying a custom made transimpedance amplifier is needed=
 to do the I-V conversion and then a voltage preamp can be used.
>=20
> So I am thinking if a commercial current preamp will also work.

It would be nice if it did, but it seems unlikely.

If your piezo-electric transducer has a mechanical resonance at or near the=
 acoustic frequency you want to pick up, you are going to have to character=
ise the transducer - drive it electrically at frequencies around the freque=
ncy of interest form around a factor of ten higher to a factor of ten lower=
, and map the complex impedance of the transducer - phase shift and amplitu=
de.

If you do it right - and the transducer hasn't got complex mix of resonant =
modes in the frequency range of interest - you should be able to set up ind=
uctance/capacitance/resistance model of the device (mode coupling can - in =
theory - be handled by using coupled inductors, but in practice, if you nee=
d to to do that you are in trouble).

The piezo-electric quartz crystals sold (in vast numbers) as frequency refe=
rences are regularly modelled in this way.

Using a transimpedance amplifier to get a signal out of such a transducer m=
ight be tricky. The impedance of the transducer becomes part of the feedbac=
k network around the amplifier, which can make it difficult to keep the amp=
lifier stable.

A voltage-follower with gain doesn't present the same kinds of problem. The=
 TL082 is relatively noisy - 18nV/per root Hz - where the OPA655 I mentione=
d is quieter - 7nV/per root Hz - if faster and a lot more expensive.

The OPA657 is a bit quieter again. You can only use it at voltage gains of =
seven or higher, but it is a lot faster as well as quieter, if just as expe=
nsive.

--=20
Bill Sloman, Sydney

On Wed, 12 Feb 2014 18:18:03 +1100, the renowned "Phil Allison"
<phil_a@tpg.com.au> wrote:

>
>Doubt he needs to go for fancy lab pre-amp units.
>
>
>
>...  Phil
>

The fancy lab pre-amp units are not much more than a JFET front end on
a low-noise op-amp. 


Best regards, 
Spehro Pefhany
-- 
"it's the network..."                          "The Journey is the reward"
speff@interlog.com             Info for manufacturers: http://www.trexon.com
Embedded software/hardware/analog  Info for designers:  http://www.speff.com