Re: piezo transducer signal amplification

Winfield wrote:
MooseFET wrote:
Winfield wrote:
[....]

You'll want lots of gain, of course, but we can
tell you the noise level will be determined by
a parameter we call voltage noise, e_n, which
is lowest for JFETs with a large die area.
If you need to go descrete:www.interfet.comand look at the
IF1801 JFET - http://www.linearsystems.comand look at the
LSK170 You can also parallel JFETs to get better noise
performance. The improvement goes as the sqrt() the number
of JFETs.

Depending on the capacitance of the crystal, the current
noise of the amplifier may also matter. The current noise
times Xc gives an additional noise.

As op-amps go, the LT1169 may be about as good as you can
do for low noise voltage and current.
http://www.linear.com/pc/downloadDocument.do?navId=H0,C1,C1154,C1009,...

The LT1169 is a nice amplifier, but it's got a
rather small JFET die area, as evidenced by its
capacitance, only 2pF. This shows, with its
moderately-high noise spec, e_n = 6nV.

As we point out in AoE's lengthy low-noise chapter,
optimum low-noise performance is had when a JFET's
Ciss is near to the sensor's capacitance. Sadly,
it's often hard to reach that goal. For example,
no opamps are available with anything like 150pF
of input capacitance, but one of the higher ones
is Analog Device's AD743, which is 20pF and has
e_n = 2.9nV, twice as good as the LT1169.

As for discrete parts, NXP's (Philips) bf862 at
10pF and 0.8nV is pretty good.

It's interesting that the bf862 JFET has half the
capacitance of AD's fine JFET opamp, but instead
of sqrt-2 more noise, it has 3.6x less noise. My
suggestion, get a lifetime supply of bf862 JFETs.


Lifetime supply? Now please don't tell me that they rang the bell on that one ...

<writing note to self: Get some more BF862>

--
Regards, Joerg

http://www.analogconsultants.com/
.

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