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Analog input : 0-XV to 0-2.5V

Hi,

I'm designing an analog input that will accept any external sensor to our device. I want to be able to support 0-3V, 0-5V, 0-10V and 0-12V.

-I only have access to a 3V regulated voltage supply on the PCB.

-I'm using the 12-bit ADC on the Msp430x1612 microcontroller.

-I'm trying to assume I don't know the output impedance of the connected sensor

My current strategy would be to have a voltage divider in front of a voltage follower.

However, the output impedance can vary very much (I'm assuming from 0 to 50 kohm). I don't see how I can have an effective voltage divider that will give me an error less than 1 LSB unless I use resitors of the order of 1000 Mohm.

I'm thinking of several solutions:

1)Changing the resistor values to accommodate each using depending on the sensor. I don't think this is very practical in terms of assembly management. 2)I could use a potentiometer in order to trim the voltage divider depending on the sensor. 3)I could give myself headroom assuming a maximum impedance of let's say 50 kohm - 2.5V. When the impendance is lower the maximum voltage would drop. It would be up to the user to adjust the range in software. This solution is very practical in terms of hardware but it diminishes the range and hence the resolution.

Any other solution to propose? I'm sure there's a fundamental strategy that is much more effective.

Thank you very much.

Yvan Newtrax Technologies

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Reply to
Yvan
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Hello Yvan,

A brute force method would be to create a weak rail of 18V or so. The MSP430 can do that by toggling an output. If you use a tiny step-up transformer you may not even need to regulate anything. Or you can turn it into a little boost converter which would use a simple inductor instead. A boost needs to be regulated but it doesn't have to be accurate.

A diode voltage multiplier is also an option and may not need to be regulated. But it would contain more parts since you'd have to generate three volts times six here.

Now you can use a nice voltage follower.

Regards, Joerg

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Reply to
Joerg

or if you have a Max232 or equivalent on the board, steal some power from that to run a voltage follower

martin

Reply to
martin griffith

Why not provide your device with a fixed input resistance - say 1M ohm ? That way you have a simple, dependable specification, similar to an oscilloscope. 1 M ohm is quite a high resistance for most sensors to drive. With a fixed input resistance, you just need a resistive divider for the input. You won't need capacitive compensation across your divider at the low frequencies involved with most sensors.

Roger

Reply to
Roger Lascelles

Why do you mean by capacitive compensation across the divider? I understand that a cable has an intrinsic impedance. Is that what you mean to compensate for it?

In order to keep it simple, I decided to make a simple voltage divider that I would adjust according to the sensor connected to it.

If I decide to change, I will definitely investigate the solutions proposed here further.

I will let you know with the test results.

Thank you,

Yvan

Reply to
Yvan

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