## PSN-L Email List Message

Subject: Re: Pendulum response and Roberts' circuit
From: ChrisAtUpw@.......
Date: Tue, 5 Dec 2006 21:13:38 EST

```In a message dated 2006/12/05, skmort@............ writes:

> OK, so which components would we change, and to what value for a 2
> Hz and a 0.222 hz (4.5 sec) system?
> I might have to break out my soldering iron, ha!
> Thanks,
> Stephen

Hi Stephen,

If f is the resonant frequency of the sensor, you make 1 / 2xPixfxCxR1
= 1.09 according to Roberts. I am not sure exactly why he chose this. It
could explain the small hump in the combined characteristic.
The Max gain is R2 / R1
The lower turnover frequency is 1 / 2xPixC1xR2 This has been set to
~1/2 the lowest response frequency in the Roberts circuit.
You need two units in series.
Roberts reported significant levels of VLF noise, so it might be a
good idea to follow up the compensation circuits with a two / four pole high pass
Butterworth filter set to about the lower desired frequency.
If you want to use the circuit at long periods, you may need to use
some CAZ opamps MAX430, LTC1150 etc. They reject 1/f noise. Otherwise LF411,
OP07, LF412 etc are OK
The optimum place to insert this circuit is between the first opamp
and the low pass filters.

How about investing in a temperature compensated soldering iron which
plugs in.....? Or make an Experimenter Board plug in version first?

Regards,

Chris Chapman
In a me=
ssage dated 2006/12/05, skmort@............ writes:

OK, so which components would w=
e change, and to what value for a 2
Hz and a 0.222 hz (4.5 sec) system?
I might have to break out my soldering iron, ha!
Thanks,
Stephen

Hi Stephen,

If f is the resonant frequency of the s=
ensor, you make 1 / 2xPixfxCxR1 =3D 1.09 according to Roberts. I am not sure=
exactly why he chose this. It could explain the small hump in the combined=20=
characteristic.
The Max gain is R2 / R1
The lower turnover frequency is 1 / 2xP=
ixC1xR2 This has been set to ~1/2 the lowest response frequency in the Rober=
ts circuit.
You need two units in series.
Roberts reported significant levels of=20=
VLF noise, so it might be a good idea to follow up the compensation circuits=
with a two / four pole high pass Butterworth filter set to about the lower=20=
desired frequency.
If you want to use the circuit at long=20=
periods, you may need to use some CAZ opamps MAX430, LTC1150 etc. They rejec=
t 1/f noise. Otherwise LF411, OP07, LF412 etc are OK
The optimum place to insert this circui=
t is between the first opamp and the low pass filters.

How about investing in a temperature co=
mpensated soldering iron which plugs in.....? Or make an Experimenter Board=20=
plug in version first?

Regards,

Chris Chapman
```