_______________Detailed Description

The MAX194 uses a successive-approximation register

(SAR) to convert an analog input to a 14-bit digital

code, which outputs as a serial data stream. The data

bits can be read either during the conversion, at the

CLK clock rate, or between conversions asynchronous

with CLK, at the SCLK rate (up to 5Mbps).

The MAX194 includes a capacitive digital-to-analog

converter (DAC) that provides an inherent track/hold

input. The interface and control logic are designed for

easy connection to most microprocessors (µPs), limiting

the need for external components. In addition to the

SAR and DAC, the MAX194 includes a serial interface, a

sampling comparator used by the SAR, ten calibration

DACs, and control logic for calibration and conversion.

The DAC consists of an array of capacitors with binary

weighted values plus one “dummy sub-LSB” capacitor

(Figure 1). During input acquisition in unipolar mode,

the array’s common terminal is connected to AGND

and all free terminals are connected to the input signal

(AIN). After acquisition, the common terminal is discon-

nected from AGND and the free terminals are discon-

nected from AIN, trapping a charge proportional to the

input voltage on the capacitor array.

The free terminal of the MSB (largest) capacitor is con-

nected to the reference (REF), which pulls the common

terminal (connected to the comparator) positive.

Simultaneously, the free terminals of all other capaci-

tors in the array are connected to AGND, which drives

the comparator input negative. If the analog input is

only pulls the comparator input slightly positive.

However, connecting the remaining capacitor’s free ter-

minals to ground drives the comparator input well

below ground, so that the comparator input is negative,

the comparator output is low, and the MSB is set high.

If the analog input is near ground, the comparator out-

put is high and the MSB is low.

Following this, the next largest capacitor is disconnect-

ed from AGND and connected to REF, and the com-

parator determines the next bit. This continues until all

bits have been determined. For a bipolar input range,

the MSB capacitor is connected to REF rather than AIN

during input acquisition, which results in an input range

14-Bit, 85ksps ADC with 10µA Shutdown

_______________________________________________________________________________________

5

______________________________________________________________Pin Description

PIN

NAME

FUNCTION

1

BP/UP/SHDN

Bipolar/Unipolar/Shutdown Input. Three-state input selects bipolar or unipolar input range, or shutdown.

0V = shutdown, +5V = unipolar, floating = bipolar.

2

CLK

Conversion Clock Input

3

SCLK

Serial Clock Input is used to shift data out between conversions. May be asynchronous to CLK.

4

VDDD

+5V Digital Power Supply

5

DOUT

Serial Data Output, MSB first

6

DGND

Digital Ground

7

EOC

End-of-Conversion/Calibration Output—normally low. Rises at beginning of conversion or calibration and

falls at the end of either. May be used as an output framing signal.

8

CS

Chip-Select Input—active low. Enables the serial interface and the three-state data output (DOUT).

9

CONV

Convert-Start Input—active low. Conversion begins on the falling edge after CONV goes low if input signal

has been acquired; otherwise, on the falling clock edge after acquisition.

10

RESET

Reset Input. Pulling RESET low places ADC in inactive state. Rising edge resets control logic and begins

calibration.

11

VSSD

-5V Digital Power Supply

12

REF

Reference Input, 0 to 5V

13

AIN

14

AGND

Analog Ground

15

VSSA

-5V Analog Power Supply

16

VDDA

+5V Analog Power Supply