Differential refernce for adc

[aashishsharma]

hey I am using max1415 adc in differential input mode. maximum Differential Input i am having is 4 volt p-p sine wave (ie 3- (-1)).
datasheet of max1415 tells me that max vref i can get (which is ref+ - ref-) is 1.75V. Now my que
1. since I can have -1V input at one pin does that mean i need to have atleast -1V at ref-? or i can simply get 0.875 and -0.875 volt at two pins ref+ and ref- respectively to get 1.75V. Ofcourse i can see that i would need to scale input down using registers to make in compatible with ADC input range which will be 1.75V. Also keeping in mind minimum voltage any pin in max1415 can take is -0.3V
2.common mode voltage for ADC input is 1V [(3-(-1))/2] does that effect common mode voltage for ref in anyway?
3. how to create differential refernce input? i just have 3.3V on board
i am really confused regarding this differential input and ref
Thnks in advance

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datashhet :http://datasheets.maximintegrated.com/en/ds/MAX1415-MAX1416.pdf

 

[iop95]

You need to reduce input voltage and shift by 1V to avoid negative values. Shift input voltage by adding a DC source of 1V, so new range will be 0 - 4V and then add a divider with factor 2, so new range will be 0 - 2V; with these range value can go to MAX1415; in software code need to take account these operation (shift and scaling). REF+ and REF- are to use an external reference, but in this case I think don't need, so remain unused and ADC will use internal reference.

 

[aashishsharma]

i dont think so iop95. Datasheet doesnt say anything about internal reference
heres link : https://datasheets.maximintegrated.com/en/ds/MAX1415-MAX1416.pdf
Also it does provide the facility to get negitive voltage. AIN range is given to be -VREF/GAIN to VREF/GAIN

 

[iop95]

Yes, sorry; don't have internal ref; you must provide a 1.225V external ref; it's simple with a TL431 or similar shunt ref. witch have good stability and temp compensation also.

 

[aashishsharma]

1.again do i need to provide 1.225 volt at REF+ and ground the ref-?
that wont be differential i guess?
2.Is it necessary to use differential references if my inputs are differential?? If yes i can provide 4 volt at ref+ and 2.775 at ref- to get vref=Ref+ - ref-=1.225 but there are many other combinations too. How to decide and how to get that differential if thats needed at all.

 

[iop95]

1. Recommanded REF range is 1 - 1.75V and absolute voltage range at REF pins are GND to VDD. So, you can choose a convenient REF value according input range and GAIN desired. For ex. Vref- may be 1V and Vref+ 2.2V from GND, so VREF=2.2-1=1.1V.
2. Yes, may use any value that respect datasheet restriction/recomandation; check datasheet page 21 where explain about influence of VREF.

 

[GUMY]

i dont think so iop95. Datasheet doesnt say anything about internal reference
heres link : https://datasheets.maximintegrated.com/en/ds/MAX1415-MAX1416.pdf
Also it does provide the facility to get negitive voltage. AIN range is given to be -VREF/GAIN to VREF/GAIN

Absolute input voltage can not exceed supply voltage. Negative voltage referes to diferntial input. Data sheet: Note 8: This common-mode voltage range is allowed, provided that the input voltage on the analog inputs does not go more positive
than (VDD + 30mV) or more negative than (GND - 30mV). Parts are functional with voltages down to (GND - 200mV),
but with increased leakage at high temperature.

 

[aashishsharma]

hey sorry for late reply
net was not working.
Thnks i think other things in my circuit are not right i need to figure them out then only i can work ADC