3.5- and 4.5-Digit, Single-Chip

ADCs with LCD Drivers

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9

Detailed Description

The MAX1491/MAX1493/MAX1495 low-power, highly

integrated ADCs with LCD drivers convert a ±2V differ-

ential input voltage (one count is equal to 100µV for the

MAX1493/MAX1495 and 1mV for the MAX1491) with a

sigma-delta ADC and output the result to an LCD. An

additional ±200mV input range (one count is equal to

10µV for the MAX1493/MAX1495 and 100µV for the

MAX1491) is available to measure small signals with

increased resolution.

These devices operate from a single 2.7V to 5.25V power

supply and offer 3.5-digit (MAX1491) or 4.5-digit

(MAX1493/MAX1495) conversion results. An internal

2.048V reference, internal charge pump and a high-accu-

racy on-chip oscillator eliminate external components.

These devices also feature on-chip buffers for the dif-

ferential input signal and external reference inputs,

allowing direct interface with high-impedance signal

sources. In addition, they use continuous internal offset

calibration, and offer >100dB of 50Hz and 60Hz line

noise rejection. Other features include data hold and

peak hold, and a low-battery monitor. The MAX1495

also performs enhanced offset calibration on demand.

Analog Input Protection

Internal protection diodes limit the analog input range

exceeds this range, limit the input current to 10mA.

Internal Analog Input/

Reference Buffers

The MAX1491/MAX1493/MAX1495 analog input/refer-

ence buffers allow the use of high-impedance signal

sources. The input buffers’ common-mode input range

allows the analog inputs and reference to range from

-2.2V to +2.2V.

Modulator

The MAX1491/MAX1493/MAX1495 perform analog-to-

digital conversions using a single-bit, 3rd-order, sigma-

delta modulator. The sigma-delta modulation converts

the input signal into a digital pulse train whose average

duty cycle represents the digitized signal information.

The modulator quantizes the input signal at a much

higher sample rate than the bandwidth of the input.

The MAX1491/MAX1493/MAX1495 modulator provides

3rd-order frequency shaping of the quantization noise

resulting from the single-bit quantizer. The modulator is

fully differential for maximum signal-to-noise ratio and

minimum susceptibility to power-supply noise. A single-

bit data stream is then presented to the digital filter for

processing, to remove the frequency-shaped quantiza-

tion noise.

Digital Filtering

The MAX1491/MAX1493/MAX1495 contain an on-chip

digital lowpass filter that processes the data stream

periods (4 x 200ms).

The MAX1491/MAX1493/MAX1495 have 25% overrange

capability built into the modulator and digital filter:

Filter Characteristics

characteristic -3dB cutoff frequency is 0.228 times the

first notch frequency (5Hz). The oversampling ratio

(OSR) for the MAX1491 is 128 and the OSR for the

MAX1493/MAX1495 is 1024.

The output data rate for the digital filter corresponds

with the positioning of the first notch of the filter’s fre-

repeated at multiples of the first notch frequency. The

100dB at these notches. For example, 50Hz is equal to

10 times the first notch frequency and 60Hz is equal to

12 times the first notch frequency.

Hz

N

z

z

()

=

()

()





















1

1

1

1

4

-

-

-N

-

Hf

N

N

f

f

f

f

m

m

()

sin

sin

=













































1

4

π

π

FREQUENCY (Hz)

50

40

30

20

10

-160

-120

-80

-40

0

-200

060