MAX192 is inactive for long periods of time, but where

intermittent bursts of high-speed conversions are

required.

External and Internal References

The MAX192 can be used with an internal or external

reference. Diode D1 shown in the

Typical Operating

Circuit ensures correct start-up. Any standard signal

diode can be used. An external reference can either be

connected directly at the VREF terminal or at the

REFADJ pin.

The MAX192’s internally trimmed 2.46V reference is

buffered with a gain of 1.678 to scale an external 2.5V

reference at REFADJ to 4.096V at VREF.

Internal Reference

The full-scale range of the MAX192 with internal reference

is 4.096V with unipolar inputs, and ±2.048V with differen-

tial bipolar inputs. The internal reference voltage is

adjustable to ±1.5% with the Reference-Adjust Circuit of

Figure 17.

External Reference

An external reference can be placed at either the

input (REFADJ) or the output (VREF) of the internal

buffer amplifier. The REFADJ input impedance is

typically 20k

Ω. At VREF, the input impedance is a

minimum of 12k

Ω for DC currents. During conversion,

an external reference at VREF must be able to deliver

up to 350µA DC load current and have an output

impedance of 10

Ω or less. If the reference has higher

output impedance or is noisy, bypass it close to the

VREF pin with a 4.7µF capacitor.

Using the buffered REFADJ input avoids external

buffering of the reference. To use the direct VREF input,

Transfer Function and Gain Adjust

Figure 15 depicts the nominal, unipolar input/output

(I/O) transfer function, and Figure 16 shows the differ-

ential bipolar input/output transfer function. Code

transitions occur halfway between successive integer

LSB

values.

Output

coding

is

binary

with

1LSB = 4.00mV (4.096V / 1024) for unipolar operation

and 1LSB = 4.00mV [(4.096V / 2 - -4.096V / 2)/1024]

for bipolar operation.

Figure 17, the Reference-Adjust Circuit, shows how to

adjust the ADC gain in applications that use the internal

reference. The circuit provides ±1.5% (±15LSBs) of

gain adjustment range.

Low-Power, 8-Channel,

Serial 10-Bit ADC

18

______________________________________________________________________________________

OUTPUT CODE

FULL-SCALE

TRANSITION

11 . . . 111

11 . . . 110

11 . . . 101

00 . . . 011

00 . . . 010

00 . . . 001

00 . . . 000

1

2

3

0

FS

FS - 3/2LSB

FS = +4.096V

1LSB = FS

1024

INPUT VOLTAGE (LSBs)

011 . . . 111

011 . . . 110

000 . . . 010

000 . . . 001

000 . . . 000

111 . . . 111

111 . . . 110

111 . . . 101

100 . . . 001

100 . . . 000

-FS

0V

DIFFERENTIAL INPUT VOLTAGE (LSBs)

+FS - 1LSB

FS = +4.096

2

1LSB = +4.096

1024

OUTPUT CODE

Figure 15. Unipolar Transfer Function, 4.096V = Full Scale

Figure 16. Differential Bipolar Transfer Function,

±4.096V / 2 = Full Scale