Mach3Maelstrom
Newbie level 4
I have a design that used to use one type of ADC, but for design reasons I must switch to a related ADC of similar pin layout (I already have the bare boards).
The problem: My design worked flawlessly with the old ADC, but I have problems with reading data from the new ADC and I need to analyze why.
Old ADC, ADS7820: **broken link removed**
New ADC, ADS8504: Link to datasheet
Summary of major differences between the two chips:
For ADS7820 (pic found on page 9 of the datasheet):
The purpose of that potentiometer-circuit is to give REF a 2.5V reference, so I installed a 2.5V source there direct to pin 3 (REF) instead. The analog signal would feed into pin 1 directly. This circuit would work flawlessly in my design as it portrays the 12-bits of information to my CPLD.
For ADS8504 (pic found on page 14 of the datasheet):
From what I understand, the datasheet mandates that I install a voltage divider (Offset) to pin 1. It still wants 2.5V on REF, so I want to still use my 2.5V reference to provide that accurately.
Two things I can't understand are:
1) Why is this voltage divider needed (the 200R and 33.2K divider)? The datasheet states it provides overvoltage protection but, as I stated above, I never go over 2.5V.
2) What voltage are they suggesting the net Offset (between the left pot & the 33.2K resistor) to be? This isn't clear to me in the datasheet how to ascertain this value. Hooking this directly into the 2.5V reference won't work as per my bench experiments.
Any help would be greatly appreciated. Thanks in advance!
The problem: My design worked flawlessly with the old ADC, but I have problems with reading data from the new ADC and I need to analyze why.
Old ADC, ADS7820: **broken link removed**
New ADC, ADS8504: Link to datasheet
Summary of major differences between the two chips:
- ADS7820 uses straight binary logic for its 12-bit output, ADS8504 uses two's complement.
- ADS7820 can only take analog values between 0V - 5V. ADS8504 can take analog values between -10V - 10V.
- My analog signal will ALWAYS be between 0V and 2.5V, so both differences above shouldn't make much of a difference
For ADS7820 (pic found on page 9 of the datasheet):
The purpose of that potentiometer-circuit is to give REF a 2.5V reference, so I installed a 2.5V source there direct to pin 3 (REF) instead. The analog signal would feed into pin 1 directly. This circuit would work flawlessly in my design as it portrays the 12-bits of information to my CPLD.
For ADS8504 (pic found on page 14 of the datasheet):
From what I understand, the datasheet mandates that I install a voltage divider (Offset) to pin 1. It still wants 2.5V on REF, so I want to still use my 2.5V reference to provide that accurately.
Two things I can't understand are:
1) Why is this voltage divider needed (the 200R and 33.2K divider)? The datasheet states it provides overvoltage protection but, as I stated above, I never go over 2.5V.
2) What voltage are they suggesting the net Offset (between the left pot & the 33.2K resistor) to be? This isn't clear to me in the datasheet how to ascertain this value. Hooking this directly into the 2.5V reference won't work as per my bench experiments.
Any help would be greatly appreciated. Thanks in advance!
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