hi everyone,i'd like a bit of advice here: i need a preamplifier for measuring verysmall vibrations on a piece of scientific kit. the sensor (a geophone -i=2Ee. a coil and magnet) will output voltages of about 0.1 microvolts(in the range 0.1Hz to 1kHz), so i need to get hold of a really lownoise preamplifier.firstly, does anyone know of a company might sell me what i want at areasonable price, i.e. less than $500 (=A3300)?also, do you think i can build it myself to these specifications? ihave some experience with op-amp design and i know the basic theory,but i'm not sure if i can actually get a good enough noise performance.thanks in advanceroko mijic

The LT1128 is a low-frequency version of the LT1028. I had not noticed the huge 275pF high-voltage on-chip capacitor, one of the largest I've seen. Perhaps Jim has seen (or used) bigger? Anyway, Roko, back to your geophone amplifier... If it's low frequencies you're interested in, the LT1128's spectral-noise density starts going up below 250Hz, an effect that's called 1/f noise. Thankfully its impact is not as dramatic as the graph, page 5, implies, because when you integrate the noise density to get the RMS noise level (and hence the peak-to-peak noise), the sqrt-BW part of the calculation reduces low frequency influence. A spectral noise of 0.9 to 2nV should be compared to the Johnson noise of associated resistors in the circuit, e_n = sqrt (4kTR) = 1.28nV for 100 ohms, and scales with sqrt R. For example your geophone coil's DC resistance may be much higher than 100 ohms. You also need to keep the value of critical gain resistors low. There's 40 pages of detailed discussion...

low noise amplifier - can i build it?

Jul 18, 2005 21 Replies

Rate this thread:

Adrian Tuddenham 21 years ago

It reduces Johnson noise, but does it have the same effect on flicker noise at LF?

~ Adrian Tuddenham ~ (Remove the ".invalid"s and add ".co.uk" to reply) www.poppyrecords.co.uk

Vote

Ian 21 years ago

This is the classic reason to use an AC driven displacement sensor rather than a DC coupled velocity sensor followed by a chopper. You get orders of magnitude better sensitivity at low frequencies, and flicker noise is no longer an issue. Unfortunately, I'd bet you don't have the freedom to mess with the sensor arrangement.

Regards Ian

Vote

Join the Discussion

Have something to add? Share your thoughts — no account required.

Didn't find your answer?

Ask the community — no account required