ADC MCP3304 howto increase input voltage up to 20V (default 5V)

Hi everyone,

sorry but i mistake and send this post too sci.electronics.misc

I'm using an ADC Microchip MCP 3304c, this converter has max input analog
voltage = 5V, but i must measure voltage up to 20V. Tested circuits are car
electronics fuel iniection components. I'm software developer so i don't
understand electronics science so good. Please help me how to connect ADC
and tested circuit. Max sample rate will be 600 count per second on one
port. MCP 3304C specification(PDF) is here:
http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1335&dDocName=en010538

Thanks, (sorry for my poor english)
Gregory

 

Hi, Greg. Your English is OK -- your post is a lot more descriptive
than many others written by native speakers.

The MCP3304 is an 8 channel/4 differential channel 13-bit SAR
(Successive Approximation Register) ADC (Analog-to-Digital Converter)
with high input impedance (<1uA) and SPI serial communications with the
uC (which is probably a PIC). The quick and dirty answer to your
question is to use a voltage divider. You might try something like
this (view in fixed font or Notepad):

___ 0-5V to ADC
o--|___|-------o-------o------o
30K 1% | |
.-. ---
| | ---
10K 1%| | |.01uF
'-' |
| |
=== ===
GND GND
(created by AACircuit v1.28.5 beta 02/06/05 www.tech-chat.de)

This will cut your 0-20VDC signal down to the 0-5VDC you're asking for.

This may not be what you need, though. First, the data sheet says that
if the input voltage goes more than 0.3V above Vdd or 0.3V below GND,
you'll have some serious problems. This may or may not be taken care
of by the largish cap, depending on the type of transient you may have
and how good your layout is.

Also, if you're using the 5VDC microcontroller supply as Vref, you
really can't expect much better than 7-bit (1 part out of 128) or 8-bit
(1 part out of 256) resolution, and your accuracy is going to be
primarily dependent on how close your 5VDC supply is to 5.000V, as well
as power supply ripple, load regulation, and drift with temperature and
time. That's why only 1% resistors are shown above. If you need
higher resolution, it's usually much better to use high precision
resistors and use an external reference such as the LM385BZ-2.5, and
then change the VDVR above to divide-by-8 instead of divide-by-4 as
shown.

If you have any more questions or need more help, feel free to post
again.

Good luck
Chris