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Author Topic: Unity Gain Amplifier / Voltage Follower - Selecting resistor  (Read 3251 times)
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I like the idea of having a wee bit o' resistance betwixt the two.
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It appears the temperature sensor on the circuit being monitored has a diode on the grnd (looks like a shottky so I think -- lightning protection?)
And the positive is a 10k resistor.

It sounds to me that the temperature sensor arrangement has quite a low source resistance, so you can connect it directly. Can you provide a link to the datasheet for the sensor?

If it's on a long lead then you might want to include a 10K resistor between the sensor and the analog input pin, and a 0.1uF capacitor between the input pin and ground.
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It sounds to me that the temperature sensor arrangement has quite a low source resistance, so you can connect it directly. Can you provide a link to the datasheet for the sensor?

If it's on a long lead then you might want to include a 10K resistor between the sensor and the analog input pin, and a 0.1uF capacitor between the input pin and ground.

Nah no datasheet for it - though it's just a NTC 25 deg C 10k sensor.

Long lead --- from the sensor to the controller - yes, from the controller to my controller? Not so much

Can't hurt to have 10k resistor and the cap - so I'll add that.

Will test it today, connect the arduino direct and measure sensor voltage before and after connection, with any luck it won't change much at all..
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No impact at all..

So does that mean the arduino and cable has a higher resistance/impedance than the jumper wires / fingers, etc that were the way?
Doesn't make sense at the moment, in a resistor divider, you have one resistor (the sensor), and the other a known value and you read the mid point of this.

What I find is if I read the voltage between the two cables that go to the sensor (i.e. think of a thermistor, you would wire it up with one side to the ground, and the other to the resistor, and measure the mid point of what you get back).

I wasn't expecting to find the same voltage between the two though - if anything, I'd have expected to find the 5V being supplied to the thermistor.
The diode on the ground side of the sensor is odd too.

Is there perhaps another reason for it to not care ?

I do recall that the controller reset / had randomness when I tried measuring once a while back, but I think I tried measuring resistance of the thermistors in circuit - this would mean I am taking current from the circuit, right?
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What it all means is that a high-impedance circuit placed in parallel with a device under test has no more effect upon the D.U.T. than a voltmeter (which is a high-impedance circuit placed in parallel with a device under test). 
[There is, in fact, (to split hairs) some negligible effect from that because it's a high or very high impedance, but not an infinite impedance.]

Back in the bad old days, voltmeters weren't the high-impedance wonders that they are today.
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