Forwarned by the work of others, I decided to see if there was an alternative to removing the 560 ohm resisitor pack in the ADIO-100 as discussed elsewhere. It turns out the choice of the PIC 18F8455 as the processor in the ADIO-100 drives the requirement for a low impenance analog source. The 18F2455 specification can be found here http://ww1.microchip.com/downloads/e...doc/39632e.pdf; Section 21.1 states "The maximum recommended impedance for analog sources is 2.5 kOhm". The rationale appears to be related to the design of the on-chip ADC circuitry.
The ADIO-100 560 Ohm voltage divider does two things, first, it drops the 10VDC maximum ADIO-100 input voltage to 5VDC which is the nominal maximum input voltage for the PIC 18F8455. Second, it guarantees the analog input impedance will be low, but, as noted by others, it loads down the analog sources such that the ADIO-100 measured voltage is not what the analog output of the sensor actually is.
An alternative to removing the resistor pack (and therefore the voltage divider) is to interpose a buffer amplifier between the analog sensors and the each ADIO-100 analog input. The buffer amplifier presents a high impedance input to the analog sensor and low impedance output to the 560 ohm voltage divider. The buffer amplifier is simply an Op-Amp configured as a voltage follower (unity gain non-inverting); in this configuration, the powered Op-Amp will pull whatever power is required so that its output voltage matches the it's input voltage from the analog sensor.
The downside of course is that you have to build this. The unregulated output of the ADIO-100 is a convienent power source for a 7810 voltage regulator to provide 10VDC to power the buffer amplifiers and anything else required; in my application I have 4 copper clad thermistors which need a power source so the 10VDC is used for them as well. The use of 10VDC also makes working on the circuitry somewhat safer with respect to the ADIO-100 (shouldn't be able to kill the analog inputs with over voltage), but makes the selection of the buffer amplifier somewhat more complicated.
Since the range of input to the buffer amplifier is 0 - 10 VDC, the desired output of the buffer amplifier is also 0 - 10VDC. However, using 10VDC as the power supply to the buffer amplifier requires the uses of a class of amplifier known as 'rail to rail' (e.g. AD820 or LMC8482). This type of amplifier can swing its output to within 20 millivolts or so of its 'rail voltages' (0 VDC and 10VDC in this case); a typical off the shelf op-amp will only be able to get within 1-2 volts of its rail voltages.
I did test the thermistor without and with the buffer amplifier; without the amplifier, the ADIO-100 measured voltage was significantly in error compare to a DVM as predicted. With the buffer amplifier the ADIO-100 measurement was on the money.
The ADIO-100 560 Ohm voltage divider does two things, first, it drops the 10VDC maximum ADIO-100 input voltage to 5VDC which is the nominal maximum input voltage for the PIC 18F8455. Second, it guarantees the analog input impedance will be low, but, as noted by others, it loads down the analog sources such that the ADIO-100 measured voltage is not what the analog output of the sensor actually is.
An alternative to removing the resistor pack (and therefore the voltage divider) is to interpose a buffer amplifier between the analog sensors and the each ADIO-100 analog input. The buffer amplifier presents a high impedance input to the analog sensor and low impedance output to the 560 ohm voltage divider. The buffer amplifier is simply an Op-Amp configured as a voltage follower (unity gain non-inverting); in this configuration, the powered Op-Amp will pull whatever power is required so that its output voltage matches the it's input voltage from the analog sensor.
The downside of course is that you have to build this. The unregulated output of the ADIO-100 is a convienent power source for a 7810 voltage regulator to provide 10VDC to power the buffer amplifiers and anything else required; in my application I have 4 copper clad thermistors which need a power source so the 10VDC is used for them as well. The use of 10VDC also makes working on the circuitry somewhat safer with respect to the ADIO-100 (shouldn't be able to kill the analog inputs with over voltage), but makes the selection of the buffer amplifier somewhat more complicated.
Since the range of input to the buffer amplifier is 0 - 10 VDC, the desired output of the buffer amplifier is also 0 - 10VDC. However, using 10VDC as the power supply to the buffer amplifier requires the uses of a class of amplifier known as 'rail to rail' (e.g. AD820 or LMC8482). This type of amplifier can swing its output to within 20 millivolts or so of its 'rail voltages' (0 VDC and 10VDC in this case); a typical off the shelf op-amp will only be able to get within 1-2 volts of its rail voltages.
I did test the thermistor without and with the buffer amplifier; without the amplifier, the ADIO-100 measured voltage was significantly in error compare to a DVM as predicted. With the buffer amplifier the ADIO-100 measurement was on the money.
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