At 09:21 AM 7/03/97 -0500, you wrote:
>At 12:57 AM 3/8/97 +1100, you wrote:
>>I wish to use a pic to determine a signal which has a resolution of 10
>>microvolts in 200 millivolts. It also has a requirement for any component to
>>be in a very small package (ie surface mount and preferably in an 8-pin
>>SOIC). I can get a 16 bit ADC but the range is usually that of the supply
>>rail to ground. If I was to look at 10 microvolts in 5 volts then a 20 bit
>>ADC would be needed. As you can see I would need serial I/O for the digital
>>side. The ideal solution would be the LTC1298 but with a resolution of 20
>bits.
>
>Peter,
>
>I regularly get 12-16 bits of precision out of an 8-bit (PIC 71 or 73/74)
>doing the following:
>
>Take your samples are _very_ regular intervals, while applying a high then
>low signal (at 50% duty cycle) such that the signal is pulled high or low
>by 1-bit of resolution (1 bit of 8).
>
>The values used for the resistor R1 and cap C1 are determined by the drive
>of your analog signal, its level, and the amplitude of the square wave you
>are providing.
>
>(The little "o" is the junction of the components).
>
>Wire this up and look at it with your scope (use a 1x probe).
>
>
>                +----R1------< square wave source
>                |
>                |
>                |
>Analog In>----o------------> input to A/D converter
>                |
>                |
>                |
>                C1
>                |
>                |
>                GND
>
>Andy
>
>==================================================================
>Andy Kunz - Montana Design - 409 S 6th St - Phillipsburg, NJ 08865
>          Hardware & Software for Industry & R/C Hobbies
>        "Go fast, turn right, and keep the wet side down!"
>==================================================================

Andy,

Thanks for this and I will certainly try this improved resolution. But with
the 16C71/74 I understand that the range input is that of the supply (5
volts) and so to get resolutions of 10 microvolts in 5 volts I need 20 bits.
If I could scale the range to read 0 to 200 millivolts then 16 bits is
enough. Alternatively I need 20 bits on an ADC.

This ADC/PIC/etc needs to be able to fit in a tube(with a pH probe,
conductivity probe and temperature sensor) which has external dimensions of
12mm diameter and 300mm long. This is why I require something small.

Tahnks again.


Peter
Neosystems