Cmos, high-speed, 8-bit adcs with multiplexer, Bipolar input operation – Rainbow Electronics MX7828 User Manual
Page 10
MX7824/MX7828
f
MAX
permits a maximum sampling rate of 50kHz per
channel when using the MAX158/MX7828 and 100kHz
per channel when using the MAX154/MX7824. These
rates are well above the Nyquist requirement of 20kHz
sampling rate for a 10kHz input bandwidth.
Bipolar Input Operation
The circuit in Figure 10a can be used for bipolar input
operation. The input voltage is scaled by an amplifier so
that only positive voltages appear at the ADC’s inputs.
An external reference should be used for the MX7824/
MX7828, but is not needed with the MAX154/MAX158.
The analog input range is ±4V and the output code is
complementary offset binary. The ideal input/output
characteristic is shown in Figure 10b.
CMOS, High-Speed, 8-Bit ADCs
with Multiplexer
10
______________________________________________________________________________________
1pF
C
S
12pF
C
S
2pF
1pF
•
•
•
15 LSB COMPARATORS
TO LS
LADDER
R
ON
1pF
1pF
•
•
•
16 MSB COMPARATORS
TO MS
LADDER
R
ON
R
MUX
R
S
V
IN
AIN1
11111111
11111110
10000010
10000001
01111111
01111110
10000000
00000001
00000000
00000010
0V
AIN INPUT VOLTAGE (LSBs)
FS = 8V
1LSB = FS / 256
11111101
+FS
2
-FS + 1LSB
2
R
S
V
IN
AIN1
C
S1
2pF
C
S2
2pF
32pF
B MUX
600
Ω
R
ON
350
Ω
MAX154
MAX158
AIN1
ONLY CHANNEL 1 SHOWN
V
REF
+
REF OUT
V
DD
V
REF
-
GND
11.5
Ω
3.57k
10.0k
0.01
µ
F
0.01
µ
F
0.1
µ
F
47
µ
F
16.2k
V
IN
+5V
CS
RDY
INT
DB0–DB7
RD
Figure 9a. Equivalent Input Circuit
Figure 10b. Transfer Function for ±4V Input Operation
Figure 9b. RC Network Model
Figure 10a. Bipolar ±4V Input Operation