I have a customer using the LTC2358-16. The device is able to accept different input voltage ranges, but in this case I am referring to the true bi-polar input. The device should operate between +10.24 and -10-24 on the inputs. They are only seeing +/- 6 Vdc and I was wondering if it had to do with the SoftSpan configuarations?.
The datasheet indicates the following;
For channels configured in SoftSpan ranges 7, 6, 3, or 2, the LTC2358-16 digitizes the differential analog input voltage (VIN+ – VIN–) over a bipolar span of ±2.5 • VREFBUF, ±2.5 • VREFBUF/1.024, ±1.25 • VREFBUF, or ±1.25 • VREFBUF/1.024, respectively, as shown in Table 1a. These SoftSpan ranges are useful for digitizing input signals where IN+ and IN– swing above and below each other.
Does this mean I am losing some of the Vin capabilities due to the digitization ??
To deal with this "offset" the customer is raising the input voltage to +/- 15 Vdc, so that they can see +/- 10.24 at the inputs.
Are they going about this the wrong way ??
If I understand your question correctly, you are saying that when your customer applies a +10.24V input voltage to the LTC2358-16, they get a code that is the equivalent to +6V (4B00h) with softspan7(+/-10.24V range) selected and REFBUF=4.096V.
The ADC should perform exactly as indicated in the datasheet. If the difference between INx+ and INx- is 10.24V, the ADC is in softspan7 and the REFBUF voltage is 4.096V, the ADC should respond with a +Full-Scale output code of 7FFFh.
If this is not the case, make sure that all unused inputs are grounded and that REFBUF measures 4.096V. Also look with an oscilloscope to make sure that the correct configuration word is being input to the ADC and that the SDO output data is being properly clocked into the FPGA or microcontroller that is being used.
If that doesn't solve the problem, provide a complete schematic including power supplies, ADC drivers, and whatever is controlling the ADC. Also include a photograph of the setup and two oscilloscope photos. The first photo showing 1 complete cycle of the data transfer (a total of 24 SCKI pulses) with CNV, SCKI, SDI and SDO for the channel of interest. The second photo showing CNV, BUSY and SCKI.
Thank you !!!
Andrew Miller MSc. , EngT, BA
Regional Analog Solutions Manager-Canada
Arrow Electronics Canada Ltd.
Engineering and Design Services Group