LTC1856 configuring and reading back differential channels

Question

I'm testing the LTC1856 differential channels, Chan 6 & 7. I'm confused how to configure them and read them back. 
 Both channels work in single ended mode reading back 32767 0x7FFF with +10V input. 
 When I configure the channels as differential and input +10V on chan 6 and -10V on chan 7 I read back 32767 from both channels. I also get the same result when injecting +5V on chan 6 and -5V on chan 7? 
 I've configured both channels with the input word 00110000 which should configure the multiplexer for differential channels + on 6 and - on 7.

ADIApproved · Accepted Answer

If you need to measure a +/-20V span, may I suggest using the LTC2358-16 and the application circuit of Figure 15 on page 11 of AN167. http://www.linear.com/docs/58894 This circuit will not work well using the LTC1856 because of its much lower input impedance.)

ADIApproved · Answer

The 8-bit SDI input word configures the LTC1856 for the next conversion, not the current conversion being shifted out. 
 +10V is the +full-scale input voltage of the LTC1856. Applying +10V on Ch6 and -10V on Ch7 is a total of 20V. This is above +full-scale, so the ADC will output +full-scale which is 32767. Applying +5V on Ch6 and -5V on Ch7 is a total of 10V which is equal to +full-scale. Again the ADC will output 32767. 
 If Ch7 is connected to +5V and Ch6 is connected to GND with an SDI input word of 00110000 I would expect the SDO output word to be 16384.

cliver1 · Answer

Hello Haresh, 
 If you make the OVDD pin 3.3V and the other supply pins AVDD and DVDD 5V then this would work.

Electronic Design Services - PartnerZone

Products Mentioned

LTC1856 configuring and reading back differential channels