Hello folks, I am currently building a digital voltmeter (3 x IN-14, 1 x IN-19A) using the 10 Bit ADC MAX1242 and the PIC16F627A as the controller. I would like to measure voltages up to 300V.
Here is what I did: - Vref was conencted to the 5V logic supply voltage, buffered with 2.2uF and 100nF. The 5V are obtained from an 78ß5 regulator that is buffered with 100nF on both sides. - Vin is taken from a voltage divider consistong of R1 = 9.970MOhm, R2 = 151kOhm, results in a factor of n = 0.0149. The resistors' values were measured within the working circuit. Addidionally, V_in has been buffered with 10nF due to the large impedance of the divider (see the datasheet of the MAX1242 here: http://datasheets.maxim-ic.com/en/ds/MAX1242-MAX1243.pdf) - I left /SHDN floating so that I could use the external 5V reference (see the datasheet). I am calculating the applied voltage as follows: U_measured = n * U_applied ^ U_measured = U_ref / 1024 * Bits <=> U_applied = U_ref / 1024 / n * Bits With U_ref = 5V and n = 0.0149 I get U_applied = 0.327 V/Bit * Bits I programmed that into my controller with a normal floating point multiplication, fixed-comma-arithmetic will be implemented during optimization later. Checking the displayed voltage with my multimeter revealed that the voltages calculated my the uC are off by the factor 2; not exactly 2, but in the range. So now I am wondering if I did something wrong in the concept, or if there is another mistake in my setup. I should also mention that the last two digits of the ADC value ripple rather strongly. Any suggestions? Thanks, Jens -- You received this message because you are subscribed to the Google Groups "neonixie-l" group. To post to this group, send an email to [email protected]. To unsubscribe from this group, send email to [email protected]. For more options, visit this group at http://groups.google.com/group/neonixie-l?hl=en-GB.
