TXMC638 User Manual Issue 1.0.2
Page 48 of 86
7.9.2 ADC digital Output Coding
Differential common mode voltage is compensated by the analog input stage. In addition, the differential
input voltage is reduced by an analogue divider which is built with a differential operational amplifier. The
feedback resistors of this operational amplifier determine the divider value. The two resistance values 4k7
3k74 build together the divider of 1.2567. With maximum ±4.096 V analogue differential input voltage range
of the ADC LTC2323-16 ensued in a theoretical maximum of ±5.1474 V differential input voltage range for a
TXMC638 analog input channel.
Due to the ADC’s true differential inputs, the ADC output coding significantly differs compared to a single
ended input.
Analogue to a single ended input, where the range setting directly describes the ground related input voltage
range, the ADC range setting describes the range of ground related voltages that can be tied to the ADC
differential inputs. This results in an extended input voltage range, since the ADC measures the voltage
between the differential inputs VIN- and VIN+.
An Example: The TXMC638 voltage range is rounded ±2.5 V, so the allowed (single ended, ground related)
voltage on each ADC input pin is ±2.5 V. When we examine the two largest differential voltages, we get
following results:
VIN-
VIN+
ADC Input Value
-2.5 V
+2.5 V
+5 V
+2.5 V
-2.5 V
-5 V
Table 7-12: ADC Data Coding Example
The example shows that the range of differential ADC input values is -5 V to +5 V, which results to a full
scale range of 10 V for the ±2.5 V ADC Input Range. Similar
The TXMC638 data coding is two’s complement.
Description
TXMC638
LTC2323-16
Digital Code
Full Scale Range
10.2948 V
8.192 V
-
Least Significant Bit
157
ȝ9
125 μV
-
Full Scale (pos.)
5.147223 V
4.095875 V
0x7FFF
FSR - 1LSB
5.147066 V
4.095750 V
0x7FFE
Mi 1LSB
157 μV
125 V
0x0001
Midscale
0.0 V
0.0 V
0x0000
Midscale – 1LSB
-157 μV
-125 μV
0xFFFF
-FSR + 1LSB
-5.147223 V
-4.095875 V
0x8001
Full Scale (neg.)
-5.147380 V
-4.096000 V
0x8000
Table 7-13: ADC Data Coding
