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# LTC2497 Differential reference

Hi

I am using LTC2497. I wanted to clarify the following,

My Vs = 5v and I am planning to use a precision 2.5V as the Vref for differential ADC. I wanted to check if this is the correct value, the last sentence states, the Vref will set the full scale range for all input channel. My concern is that upon using the 2.5Vref, I can only read +/- 1.25V.

Thank you.

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• Hi..I biased my IN- at 2.5v and was only able to get +/- approx 0.5V.

According to your earlier statement, I can get 2x range with a lower bias voltage? E.g. IN- at 1.25v and IN+ will be in the range of 0-2.5v?

Thank you.

• I have attached my schematics as well. Appreciate if you could point out my error.

Thank you.

• Your schematic shows that REF- is floating.

• Hi

I managed to solder the REF- to GND. However, I am still facing some issue with the conversion...I am getting 1/4 of the expected value...The details of my setup are as follows,

1. Driven by: Battery

2. REF+ = VSS, REF- = GND

3. Differential ADC, Vref = Vss/2

Anything else i could have missed?

Thank you.

• 1. What is driven by battery? What is the battery voltage? Is the battery voltage attenuated or filtered in any way?

2. Your schematic shows REF+ = 5V. Is that what you mean by VSS?

3. Do you mean the -input of the ADC is connected to 5V/2?

you say you are getting 1/4 of expected value. What is the input voltage applied? what is the output code provided by the ADC?

• 1. We are driving a E-Load. Battery voltage 3.3V.

2. Vss = REF+ = 5V from USB

3. REF- = 5V from USB / 2

4. Applied input voltage = 5v

5. Output code by ADC: 100101010101000110000000

• Voltage applied = 3.062v

• Looking at Table 1 of the data sheet, you will see that the two LSBs are for overrange and sign. That is followed by 16 data bits and 6 zeros. With the data you provided that decodes to 5546Hex. Converting that to decimal is 21380. 21380/65535 * 2.5 = 0.8327V That is relative to the 2.5V that the -input is setting at, which makes the input voltage 3.3327V. That is still a sizeable error from 3.062V but if you are basing your reference voltage on the 5V from the USB, that much error is quite possible. Measure the voltages at REF+ and IN- and see if the number you are getting makes sense.

• Hi ghoover

I managed to get the voltage reading for anything below 2.5v. Anything above 2.5v gives me "11" and the reading is out of range. Any possible thing i could have missed?

1. We are driving a E-Load. Battery voltage 3.3V.

2. Vss = REF+ = 5V from USB

3. REF- = 5V from USB / 2

4. Applied input voltage = 5v

5. Output code by ADC: 1111111100000111000000

Thanks!

• I measured the REF+ and REF- its at 4.799v and 0v.