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Amidon:
Because a 2.2uF cap isn't going to muffle RF real good and the 22uF is probably some large size cap, so not good for RF either. After all, you don't want it to function nicely only to find out that it all falls off the cliff when someone walks up with a GSM phone. Those can be pretty tough.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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I originally was using a 1nF instead of the 2.2uF but some recommended the 2.2uF because I wanted a cutoff of around 15Hz. So 2.2uF was the calculated fc with the 4.7kOhm resistor I am using. However, according to Analog Devices reference, they indicated a cap between the -/+ input that was on the order of 10 x the filter caps. They indicated that this was to help with the possible mismatch of the two
2.2uF caps hence the 22uF cap there.So, would it be better to just replace the 2.2uF with 0.1uF instead? Or would it be best to have both, because they actual function independantly but the total effect is the parrallel combination. If memory serves correctly.
Regards,
James
Glenn Møller-Holst wrote: ...
...
Hi Jim
If you only need to transport the measurements 1-2 meters in a protected environment, then you can use I2C or Controller Area Network (CAN):
Here you have a digital servo with the possibility of intelligence and I2C communication:
Schematic:
Softwaren in C:
Controller Area Network (CAN):
regards,
Glenn
Glenn Møller-Holst wrote: ...
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Glenn
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I didn't even think of the output. A 100 ohm would be a good idea. The transducer can't be mounted close to the circuit because it is going to be used to detect the force exerted by an embryonic chicken. I.E. it is for an experiment in Biomed resource, in which a 9 day egg is broken open and force measures are being taken off of the embryo's limbs. The data is collected and used to validate someones thesis on nueral response and development. I don't unstand all of it, I am just here to build the required equipment for the experiments.
The need of changing the gain is to deal with different stages of the development of the embryo. I.E. A 15 day will exert more force and thus possibly saturate the amp. The transducer itself is rate for
10grams of force and can be changed by spring constants.I originally was going to use a digital potentiameter but did not find any with such low resistance ranges, hence the idea of a dip switch or rotary switch to select gain.
I will be using a microprocess later in the development for quick non graph force readouts. and the at some time include a color TFT and provide live graphing without the need of a computer, but for now, the computer is the first step. The researcher needs to show progression, so I have to start with small pieces so he can collect data during the process for is requirements. For example, the first version of this was to determine how much force was actually being exerted by the 9 day embryo and we used the indicated tranduces. The first thing I did was build a diff amp from a 741 genreal op amp. It gave the initial conditions of force so we could determine appropriate force transducer required and gains of amp. So this is the second revision. I did have the same problems with the 741 and DC level shift when moving around, but for initial setup it was not an issue because we were able to get the preliminary data to better understand the problem and build the correct system for what was being measured.
James
I am presuming that you have mounted the whole test assembly on a rubber pad and not just the transducer (which will insulate some of the forces you are trying to measure).
It is worth using shielded twisted pair cable with low microphony characteristics. Looks like the transducers have a decent connector arrangement so you should carry the shield into the connector from the instrumentation amplifiers (but keep the individual shields separate at the connector).
In a system I deal with we have to make measurements from systems whose senosrs are in the midst of magnetic fields in 10's of Tesla, where millions of amps flow in (effectively) a single turn secondary and where high voltages and currents are used in abundance. We also run a lot of vacuum pumping and have enormous temperature differences. There are three reference earth systems and the difference between them can exceed
75V at points during the experimental runs. This is despite the fact that all three earth systems do connect together at one star-point. With signals we may get from some of the sensors being in the microvolt range you can see why we learn to pay a great deal of attention to the earthing arrangements and screening.Since the last message I also note that your amplifier circuit is open to some interference problems. Again, decent layout and screening is vital with suitable attention paid to the details. I would look at the CMOS amplifiers, like OPA336, for low noise more than the bi-polar ones like the 741. Even then ensure you use decent layout of the board, appropriate screening of the circuit, and decent decoupling of the power supplies.
It all seems like a lot of work but it is worth it to get the best signals possible.
-- ******************************************************************** Paul E. Bennett............... Forth based HIDECS Consultancy Mob: +44 (0)7811-639972 Tel: +44 (0)1235-811095 Going Forth Safely ..... EBA. www.electric-boat-association.org.uk.. ********************************************************************
example:
space:
Amidon:
Use both. The 2.2uF provides a nice rolloff against low frequency noises like harmonics from a computer power supply, the 0.1uF cap muffle noise from VHF and FM stations. Yes, they function almost independently. Just like all the vitamins in our body. If only one is missing we get sick ;-)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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I am now going to add some ferrite beads that are surface mount. I was going to use 0805. What DC resistance should it be? What Impedance?
I'm not sure of this, but I would guess that the DC resistance should be as low as possible. and the Impedance should be high for large frequencies.
The ones I was looking at from Digikey are:
-1-ND
example:
space:
Amidon:
DC resistance doesn't really matter on data lines and with beads it's always very low. Impedance should be as high as possible. On lines where you have the 4.7K all the beads do is prevent some of the RF to enter the enclosure. When using beads in power supply lines you need to watch out that the saturation current is less than the maximum you'll ever draw. Not that they blow up but when they saturate they don't work anymore, they become like a piece of wire.
For optimum performance a bead needs to be followed by a 0.1uF or smaller capacitor immediately after the bead. However, in your case all you need to pretect is the opamp so it should suffice if you have 0.1uF caps right there.
I'd also look at low frequencies, to make sure there is some impedance left down there. This depends on the ferrite material. Typical noise sources are, for example, old pager systems at 27MHz or induction heaters at 13.56MHz.
They are pretty good. Have to be, because they are from my old home country :)
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
P.S.: How about these for the signal lines?
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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As for the 0.1uF caps, I hope that you are referring that they go in parrallel with the beads, and not in series. Because if they are in series, then will block the DC force signal changes. Just correct me if I misunder stood.
James
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.Yes, that would be better. But I just was wondering, what kind of kind of DC resistance tolerance these have and how they will effect the balance of the bridge itself. Never thought about that until now. But then again, it may not matter because it would be in series with the 4.7K ohm lowpass filter resistors.
I now of the board laid out and getting ready to generate the gerber files for the board house. I will post to my site later.
James
example:
space:
Amidon:
Nope, I meant from circuit side of the bead to the ground plane. Like a lowpass.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
Essentially what goes through the bead is like a very short piece of wire, it's really low in resistance.
-- Regards, Joerg http://www.analogconsultants.com/ "gmail" domain blocked because of excessive spam. Use another domain or send PM.
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