MAX1221 datasheet(21/44 Pages) MAXIM

21 / 44 page

12-Bit, Multichannel ADCs/DACs with FIFO,

Temperature Sensing, and GPIO Ports

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the setup register. See Figures 3 and 4 for the transfer-

function graphs. Program a pair of analog inputs for dif-

ferential operation by writing a one to the appropriate bit

of the bipolar- or unipolar-mode register. Unipolar mode

sets the differential input range from 0 to VREF1. A nega-

tive differential analog input in unipolar mode causes the

digital output code to be zero. Selecting bipolar mode

sets the differential input range to ±VREF1 / 2. The digital

output code is binary in unipolar mode and two’s com-

plement in bipolar mode.

In single-ended mode, the MAX1221/MAX1223/

MAX1343 always operate in unipolar mode. The analog

inputs are internally referenced to AGND with a full-scale

input range from 0 to the selected reference voltage.

Analog Input (T/H)

The equivalent circuit of Figure 2 shows the ADC input

architecture of the MAX1223. In track mode, a positive

input capacitor is connected to AIN0–AIN11 in single-

ended mode and AIN0, AIN2, AIN4–AIN10 in differential

mode. A negative input capacitor is connected to AGND

in single-ended mode or AIN1, AIN3, AIN5–AIN11 in dif-

ferential mode. The MAX1221/MAX1343 feature eight

analog input channels (AIN0–AIN7). In track mode, a pos-

itive input capacitor is connected to AIN0–AIN7 in single-

ended mode and to AIN0, AIN2, AIN4, and AIN6 in

differential mode. A negative input capacitor is connected

to AGND in single-ended mode or to AIN1, AIN3, AIN5,

and AIN7 in differential mode. For external T/H timing, use

clock mode 01. After the T/H enters hold mode, the differ-

ence between the sampled positive and negative input

voltages is converted. The input capacitance charging

rate determines the time required for the T/H to acquire

an input signal. If the input signal’s source impedance is

high, the required acquisition time lengthens.

Any source impedance below 300Ω does not signifi-

cantly affect the ADC’s AC performance. A high-imped-

ance source can be accommodated either by

lengthening tACQ (only in clock mode 01) or by placing

a 1µF capacitor between the positive and negative ana-

log inputs. The combination of the analog-input source

impedance and the capacitance at the analog input cre-

ates an RC filter that limits the analog input bandwidth.

Input Bandwidth

The ADC’s input-tracking circuitry has a 1MHz small-sig-

nal bandwidth, making it possible to digitize high-speed

transient events and measure periodic signals with

bandwidths exceeding the ADC’s sampling rate by

using undersampling techniques. Anti-alias prefiltering

of the input signals is necessary to avoid high-frequency

signals aliasing into the frequency band of interest.

Analog Input Protection

Internal electrostatic-discharge (ESD) protection diodes

clamp all analog inputs to AVDD and AGND, allowing

the inputs to swing from (AGND - 0.3V) to (AVDD +

0.3V) without damage. However, for accurate conver-

sions near full scale, the inputs must not exceed AVDD

by more than 50mV or be lower than AGND by 50mV. If

an analog input voltage exceeds the supplies, limit the

input current to 2mA.

Internal FIFO

The MAX1221/MAX1223/MAX1343 contain a first-

in/first-out (FIFO) buffer that holds up to 16 ADC results

plus one temperature result. The internal FIFO allows

the ADC to process and store multiple internally

clocked conversions and a temperature measurement

without being serviced by the serial bus.

If the FIFO is filled and further conversions are request-

ed without reading from the FIFO, the oldest ADC

results are overwritten by the new ADC results. Each

result contains 2 bytes, with the MSB preceded by four

leading zeros. After each falling edge of

CS, the oldest

available pair of bytes of data is available at DOUT,

MSB first. When the FIFO is empty, DOUT is zero.

The first 2 bytes of data read out after a temperature

measurement always contain the 12-bit temperature

result, preceded by four leading zeros, MSB first. If anoth-

er temperature measurement is performed before the first

temperature result is read out, the old measurement is

overwritten by the new result. Temperature results are in

degrees Celsius (two’s complement), at a resolution of 8

AIN0–AIN11

(SINGLE-ENDED),

AIN0, AIN2,

AIN4–AIN10

(DIFFERENTIAL)

COMPARATOR

HOLD

ACQ

HOLD

ACQ

HOLD

AVDD / 2

REF1

AGND

CIN+

CIN-

DAC

AGND

(SINGLE-ENDED),

AIN1, AIN3,

AIN5–AIN11

(DIFFERENTIAL)

Figure 2. MAX1223 Equivalent Input Circuit

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