Transfer Function

Figures 8, 9, and 10 show the MAX1157/MAX1159/

MAX1175’s output transfer functions. The MAX1159

and MAX1175 outputs are coded in offset binary, while

the MAX1157 is coded on standard binary.

Input Buffer

Most applications require an input buffer amplifier to

achieve 14-bit accuracy and prevent loading the

source. Switch the channels immediately after acquisi-

tion, rather than near the end of or after a conversion

when the input signal is multiplexed. This allows more

time for the input buffer amplifier to respond to a large

step-change in input signal. The input amplifier must

have a high enough slew rate to complete the required

output voltage change before the beginning of the

acquisition time. Figure 11 shows an example of this

circuit using the MAX427.

Figures 12a and 12b show how the MAX1175 and

MAX1159 analog input current varies depending on

whether the chip is operating or powered down. The

part is fully powered down between conversions if the

voltage at R/C is set high during the second falling

edge of CS. The input current abruptly steps to the

powered up value at the start of acquisition. This step

in the input current can disrupt the ADC input, depend-

ing on the driving circuit’s output impedance at high

frequencies. If the driving circuit cannot fully settle by

the end of acquisition time, the accuracy of the system

can be compromised. To avoid this situation, increase

the acquisition time, use a driving circuit that can settle

up by setting the voltage at R/C low during the second

falling edge of CS.

Layout, Grounding, and Bypassing

For best performance, use printed circuit (PC) boards.

Do not run analog and digital lines parallel to each

other, and do not lay out digital signal paths under-

neath the ADC package. Use separate analog and dig-

ital ground planes with only one point connecting the

two ground systems (analog and digital) as close to the

device as possible.

Route digital signals far away from sensitive analog and

reference inputs. If digital lines must cross analog lines,

do so at right angles to minimize coupling digital noise

14-Bit, 135ksps, Single-Supply ADCs with

Bipolar Analog Input Range

______________________________________________________________________________________

11

Figure 9. MAX1159 Transfer Function

OUTPUT CODE

-8192

+8192

INPUT VOLTAGE (LSB)

INPUT RANGE = -10V TO +10V

0

-8190

-8191 -8189

+8191

+8190

-1

+1

11 . . . 1111

11 . . . 1110

11 . . . 1101

10 . . . 0001

10 . . . 0000

01 . . . 1111

00 . . . 0011

00 . . . 0010

00 . . . 0001

00 . . . 0000

FULL-SCALE RANGE

(FSR) = +20V

1LSB =

16384 x 4.096

FULL-SCALE

TRANSITION

Figure 10. MAX1175 Transfer Function

OUTPUT CODE

-8192

+8192

INPUT VOLTAGE (LSB)

INPUT RANGE = -5V TO +5V

0

-8190

-8191 -8189

+8191

+8190

-1

+1

11 . . . 1111

11 . . . 1110

11 . . . 1101

10 . . . 0001

10 . . . 0000

01 . . . 1111

00 . . . 0011

00 . . . 0010

00 . . . 0001

00 . . . 0000

FULL-SCALE RANGE

(FSR) = +10V

1LSB =

16384 x 4.096

FULL-SCALE

TRANSITION

Figure 8. MAX1157 Transfer Function

OUTPUT CODE

16384

INPUT VOLTAGE (LSB)

INPUT RANGE = 0 TO +10V

2

1

03

16383

16382

11 . . . 111

11 . . . 110

11 . . . 101

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

FULL-SCALE RANGE

(FSR) = +10V

1LSB =

16384 x 4.096

FULL-SCALE

TRANSITION