Rainbow Electronics MAX1314 - Manual (Page 28)

MAX1304–MAX1306/MAX1308–MAX1310/MAX1312–MAX1314
8-/4-/2-Channel, 12-Bit, Simultaneous-Sampling ADCs
with ±10V, ±5V, and 0 to +5V Analog Input Ranges
28 ______________________________________________________________________________________
Bipolar ±5V Devices
Table 6 and Figure 13 show the two’s complement trans-
fer function for the ±5V input range MAX1308/MAX1309/
MAX1310. The FSR is four times the voltage at REF. The
internal +2.5V reference gives a +10V FSR, while an
external +2V to +3V reference allows an FSR of +8V to
+12V respectively. Calculate the LSB size using:
which equals 2.44mV when using a 2.5V reference.
The input range is centered about V MSV . Normally,
MSV = AGND, and the input is symmetrical about zero.
For a custom midscale voltage, drive MSV with an
external voltage source. Noise present on MSV directly
couples into the ADC result. Use a precision, low-drift
voltage reference with adequate bypassing to prevent
MSV from degrading ADC performance. For maximum
FSR, do not violate the absolute maximum voltage rat-
ings of the analog inputs when choosing MSV.
Determine the input voltage as a function of V REF ,
V MSV , and the output code in decimal using:
V CH_ = LSB x CODE 10 + V MSV
1
4
2 12
LSB
x V
REF
=
Table 6. ±5V Bipolar Code Table
TWO’s
COMPLEMENT
DIGITAL OUTPUT
CODE
DECIMAL
EQUIVALENT
DIGITAL OUTPUT
CODE
(CODE 10 )
INPUT VOLTAGE
(V)
V REF = +2.5V
V MSV = 0
0111 1111 1111 =
0x7FF
+2047 +4.9988 ± 0.5 LSB
0111 1111 1110 =
0x7FE
+2046 +4.9963 ± 0.5 LSB
0000 0000 0001 =
0x001
+1 +0.0037 ± 0.5 LSB
0000 0000 0000 =
0x000
0 +0.0012 ± 0.5 LSB
1111 1111 1111 =
0xFFF
-1 -0.0012 ± 0.5 LSB
1000 0000 0001 =
0x801
-2047 -4.9963 ± 0.5 LSB
1000 0000 0000 =
0x800
-2048 -4.9988 ± 0.5 LSB
( )
4 x V REF
4 x V
REF
2 12
1 LSB =
TWO'S COMPLEMENT BINAR
Y OUTPUT CODE
-2048 -2046 +2047 +2045
0x800
0x801
0x802
0x803
0x7FF
0x7FE
0x7FD
0x7FC
0xFFF
0x000
0x001
-1 0 +1
(MSV)
INPUT VOLTAGE (V CH_ - V MSV IN LSBs)
Figure 13. ±5V Bipolar Transfer Function