Rainbow Electronics MAX1326 - Manual (Page 20)

MAX1316–MAX1318/MAX1320–MAX1322/MAX1324–MAX1326
8-/4-/2-Channel, 14-Bit, Simultaneous-Sampling ADCs
with ±10V, ±5V, and 0 to +5V Analog Input Ranges
20 ______________________________________________________________________________________
No other digital system ground should be connected to
this single-point analog ground. The ground return to
the power supply for this ground should be low imped-
ance and as short as possible for noise-free operation.
High-frequency noise in the V
DD
power supply may
affect the high-speed comparator in the ADC. Bypass
these supplies to the single-point analog ground with
0.1µF and 2.2µF bypass capacitors close to the device.
If the +5V power supply is very noisy, a ferrite bead can
be connected as a lowpass filter, as shown in Figure 8.
Transfer Functions
Bipolar ±10V Devices
Table 5 and Figure 9 show the two’s complement trans-
fer function for the MAX1324/MAX1325/MAX1326 with a
±10V input range. The full-scale input range (FSR) is
eight times the voltage at REF. The internal +2.500V ref-
erence gives a +20V FSR, while an external +2V to +3V
reference allows an FSR of +16V to +24V, respectively.
Calculate the LSB size using the following equation:
This equals 1.2207mV with a +2.5V internal 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, be careful not to violate the absolute maximum
voltage ratings of the analog inputs when choosing
V
MSV .
Determine the input voltage as a function of V REF ,
V MSV , and the output code in decimal using the follow-
ing equation:
Bipolar ±5V Devices
Table 6 and Figure 10 show the two’s complement
transfer function for the MAX1320/MAX1321/MAX1322
with a ±5V input range. The FSR is four times the volt-
age at REF. The internal +2.500V 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 the following equation:
This equals 0.6104mV when using the internal reference.
LSB
V
REF =
×
4
2
14
V LSB CODE V
CH MSV _
= × +
10
LSB
V
REF =
× 8
2
14
Figure 9. ±10V Bipolar Transfer Function
8 x V REF
8 x V REF
8 x V REF
2 14
1 LSB =
TWO'S COMPLEMENT BINAR
Y OUTPUT CODE
-8192 -8190 +8191 +8189
0x2000
0x2001
0x2002
0x2003
0x1FFF
0x1FFE
0x1FFD
0x1FFC
0x3FFF
0x0000
0x0001
-1 0 +1
(MSV)
INPUT VOLTAGE (V CH_ - V MSV IN LSBs)
Table 5. ±10V Bipolar Code Table
TWO’S COMPLEMENT
BINARY OUTPUT CODE
DECIMAL
EQUIVALENT
OUTPUT
(CODE 10 )
INPUT
VOLTAGE (V)
(V REF = 2.5V,
V MSV = 0V)
01 1111 1111 1111
0x1FFF
8191
9.9994
±0.5 LSB
01 1111 1111 1110
0x1FFE
8190
9.9982
±0.5 LSB
00 0000 0000 0001
0x0001
1
0.0018
±0.5 LSB
00 0000 0000 0000
0x0000
0
0.0006
±0.5 LSB
11 1111 1111 1111
0x3FFF
-1
-0.0006
±0.5 LSB
10 0000 0000 0001
0x2001
-8191
-9.9982
±0.5 LSB
10 0000 0000 0000
0x2000
-8192
-9.9994
±0.5 LSB