2.6.2 - Converting Binary Readings to Voltages [U3 Datasheet]

This information is only needed when using low-level functions and other ways of getting binary readings. Readings in volts already have the calibration constants applied. The UD driver, for example, normally returns voltage readings unless binary readings are specifically requested.

Subsections

Following are the nominal input voltage ranges for the low-voltage analog inputs. This is all analog inputs on the U3-LV, and AIN4-AIN15 on the U3-HV.

Table 2.6.2-1. Nominal Analog Input Voltage Ranges for Low-Voltage Channels

Table 2.6.2-2. Nominal Analog Input Voltage Ranges for High-Voltage Channels

Note that the minimum differential input voltage of -2.44 volts means that the positive channel can be as much as 2.44 volts less than the negative channel, not that a channel can measure 2.44 volts less than ground. The voltage of any low-voltage analog input pin, compared to ground, must be in the range -0.3 to +3.6 volts.

The “special” range (0-3.6 on low-voltage channels and -10/+20 volts on high-voltage channels) is obtained by doing a differential measurement where the negative channel is set to the internal Vref (2.44 volts). For low-voltage channels, simply do the low-voltage differential conversion as described below, then add the stored Vref value. For high-voltage channels, do the same thing, then multiply by the proper high-voltage slope, divide by the single-ended low-voltage slope, and add the proper high-voltage offset. The UD driver handles these conversions automatically.

Although the binary readings have 12-bit resolution, they are returned justified as 16-bit values, so the approximate nominal conversion from binary to voltage is:

Volts(uncalibrated) = (Bits/65536)*Span (Single-Ended)

Volts(uncalibrated) = (Bits/65536)*Span – Span/2 (Differential)

Binary readings are always unsigned integers.

Where span is the maximum voltage minus the minimum voltage from the tables above. The actual nominal conversions are provided in the tables below, and should be used if the actual calibration constants are not read for some reason. Most applications will use the actual calibrations constants (Slope and Offset) stored in the internal flash.

Volts = (Slope * Bits) + Offset

Since the U3 uses multiplexed channels connected to a single analog-to-digital converter (ADC), all low-voltage channels have the same calibration for a given configuration. High-voltage channels have individual scaling circuitry out front, and thus the calibration is unique for each channel.

See the Low-level Functionality section for detail about the location of the U3 calibration constants.