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. 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 and Reference

10

______________________________________________________________________________________

1pF

12pF

2pF

1pF

•

•

•

15 LSB COMPARATORS

TO LS

LADDER

1pF

1pF

•

•

•

16 MSB COMPARATORS

TO MS

LADDER

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

MAX154

MAX158

RD

RDY

*A2 ON MAX158.

CS

A1

A2*

A0

5V

DATA BUS

A15

A0

ZBO

MREQ

WAIT

RD

DB0-DB7

D0-D7

ADDRESS

DECODE

EN

5k

ADDRESS BUS

AIN1

2pF

2pF

32pF

B MUX

600

Ω

350

Ω

MAX154

MAX158

AIN1

ONLY CHANNEL 1 SHOWN

REFOUT

GND

11.5

Ω

3.57k

10.0k

0.01

µF

0.01

µF

0.1

µF47µF

16.2k

+5V

CS

RDY

INT

DB0-DB7

RD

Figure 9a. Equivalent Input Circuit

Figure 10b. Transfer Function for ±4V Input Operation

Figure 11. Simple Mode 0 Interface

Figure 9b. RC Network Model

Figure 10a. Bipolar ±4V Input Operation