External Reference

An external reference can be placed at either the input

(REFADJ) or the output (REF) of the MAX1178/

MAX1188s’ internal buffer amplifier. Using the buffered

REFADJ input makes buffering the external reference

unnecessary. The input impedance of REFADJ is typi-

cally 5k

Ω. The internal buffer output must be bypassed

at REF with a 10µF capacitor.

Directly drive REF using an external 3.8V to 4.2V refer-

ence. During conversion, the external reference must

be able to drive 100µA of DC load current and have an

output impedance of 10

Ω or less.

For optimal performance, buffer the reference through

an op amp and bypass REF with a 10µF capacitor.

Consider the MAX1178/MAX1188s’ equivalent input

noise (0.6 LSB) when choosing a reference.

Reading the Conversion Result

EOC is provided to flag the µP when a conversion is

complete. The falling edge of EOC signals that the data

is valid and ready to be output to the bus. D0–D15 are

the parallel outputs of the MAX1178/MAX1188. These

tri-state outputs allow for direct connection to a micro-

controller I/O bus. The outputs remain high impedance

during acquisition and conversion. Data is loaded onto

the output bus with the third falling edge of CS with R/C

back to high impedance. The MAX1178/MAX1188 then

wait for the next falling edge of CS to start the next con-

version cycle (Figure 2).

HBEN toggles the output between the high/low byte. The

low byte is loaded onto the output bus when HBEN is

low, and the high byte is on the bus when HBEN is high.

Transfer Function

Figures 8 and 9 show the MAX1178/MAX1188 output

transfer functions. The MAX1178 and MAX1188 outputs

are coded in offset binary.

Input Buffer

Most applications require an input buffer amplifier to

achieve 16-bit accuracy and prevent loading the

source. When the input signal is multiplexed, switch the

channels immediately after acquisition, rather than near

the end of, or after, a conversion. This allows more time

for the input buffer amplifier to respond to a large step

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

Bipolar Analog Input Range

10

______________________________________________________________________________________

+5V

68k

Ω

100k

Ω

150k

Ω

0.1

µF

REFADJ

MAX1178

MAX1188

OUTPUT CODE

FULL-SCALE

TRANSITION

FULL-SCALE RANGE (FSR) = +20V

MAX1188

INPUT RANGE = -10V TO +10V

1 LSB =

65,536 x 4.096

1111 1111 1111 1111

-32,68

+32,768

INPUT VOLTAGE (LSB)

1111 1111 1111 1110

1111 1111 1111 1101

0000 0000 0000 0011

0000 0000 0000 0010

0000 0000 0000 0001

0000 0000 0000 0000

1000 0000 0000 0000

0001 1111 1111 1111

1000 0000 0000 0001

0

-32,766

-32,767 -32,765

+32,767

+32,766

-1

+1

Figure 7. MAX1178/MAX1188 Reference-Adjust Circuit

OUTPUT CODE

FULL-SCALE

TRANSITION

FULL-SCALE RANGE (FSR) = +1V

MAX1178

INPUT RANGE = -5V TO +5V

1 LSB =

65,536 x 4.096

11 1111 1111 1111

-32,768

+32,768

INPUT VOLTAGE (LSB)

11 1111 1111 1110

11 1111 1111 1101

00 0000 0000 0011

00 0000 0000 0010

00 0000 0000 0001

00 0000 0000 0000

10 0000 0000 0000

01 1111 1111 1111

10 0000 0000 0001

0

-32,766

-32,767 -32,765

+32,767

+32,766

-1

+1

Figure 8. MAX1178 Transfer Function

Figure 9. MAX1188 Transfer Function