Voltage reference for 16F628 ADC

Hello,

I am looking to replace an Analog Devices (Part# REF10 ) 10 volt voltage reference device in an existing circuit. The chip supplies 10 volts to a number of n/o and n/c switch contacts that have a shunt and end-of-line resistors fitted to them. The ADC of a PIC 16F628 then measures the circuit condition through a couple of divider resistors. The REF10 would be ideal but for the fact that the switch resistors are low values and as such draw more current than the it can comfortably supply. I have thought about using a simple arrangement of a 7809 1 amp voltage regulator to supply the 10 volts and using a resistor in series with its ground lead to 'tweak' the output up to 10 volts above ground. My only worry is that the output voltage might be a bit noisy which could upset the PIC's ADC converter.

Does anyone think this could be a problem ?

TIA,

Colin

Reply to
Colin MacDougall
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I don't think it will be a problem, but it depends on the voltage range for each switch setting and the tolerance of the resistors. I am not clear about your switch/resistor arrangement, but I am assuming it is something like an R/2R resistor ladder to give you distinct voltages for each combination of switch settings.

Even if the 7809 has some noise on the output, you can remove that by adding a capacitor. It would be most effective if added at the ADC input. Just be sure to pick a value that will not slow the settling time too much to distort your ADC readings.

Or can you use larger resistors on the switches? 1 Amp is a lot of current for reading switches.

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Rick "rickman" Collins

rick.collins@XYarius.com
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rickman

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