Author Topic: Differential ADC input range  (Read 1304 times)

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Online paulcaTopic starter

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Differential ADC input range
« on: January 02, 2018, 10:54:09 pm »
If I have a differential ADC such as this one:
http://www.ti.com/lit/ds/symlink/ads1115.pdf

Can I supply voltages higher than VDD/VCC on the two pins as long as the differential is less than VDD+0.3V?

EG:
Vcc: 5V
AIN0:  8.4V
AIN1:  11.1V

And configure AIN0/AIN1 as a differential pair.  Will it read for 3.7V or will it let the smoke out?
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Offline retrolefty

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Re: Differential ADC input range
« Reply #1 on: January 02, 2018, 11:07:30 pm »
Analog pins must not be beyond GND – 0.3 VDD + 0.3. While the input can be configured for differential between two input pins this does not me they are bipolarity inputs, negative voltages are not allowed.

This is more a digital signed mode Vs a digital integer mode. It's subtle and often missed.

Quote
10.1.2 Single-Ended Inputs
The ADS1113 and ADS1114 can measure one, and the ADS1115 up to four, single-ended signals. The
ADS1113 and ADS1114 can measure single-ended signals by connecting AIN1 to GND externally. The
ADS1115 measures single-ended signals by appropriate configuration of the MUX[2:0] bits in the Config register.
Figure 40 shows a single-ended connection scheme for ADS1115. The single-ended signal ranges from 0 V up
to positive supply or +FS, whichever is lower. Negative voltages cannot be applied to these devices because the
ADS111x can only accept positive voltages with respect to ground. The ADS111x do not lose linearity within the
input range.
The ADS111x offer a differential input voltage range of ±FSR. Single-ended configurations use only one-half of
the full-scale input voltage range. Differential configurations maximize the dynamic range of the ADC, and
provide better common-mode noise rejection than single-ended configurations.
 

Online paulcaTopic starter

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Re: Differential ADC input range
« Reply #2 on: January 02, 2018, 11:23:59 pm »
Aww well, it means external diff amps then.  No big deal, thought I had found a short cut.
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Current Open Projects:  STM32F411RE+ESP32+TFT for home IoT (NoT) projects.  Child's advent xmas countdown toy.  Digital audio routing board.
 

Offline danadak

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Re: Differential ADC input range
« Reply #3 on: January 02, 2018, 11:41:36 pm »
You can tie an R to Vcc to input and one from that input to signal,
a third to ground, and offset the voltage seen by A/D. That will allow
handling a negative signal. Attached a calculator to help selecting
them. Be careful of input Z structure of A/D when you choose values.

This is one simple method. It has errors due to non precision of Vcc
(use a reference) and R tolerance (use precision Rs).

Regards, Dana.
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