ADS8322

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SBAS215

limited between –0.1V and 0.5V, allowing the input to reject

small signals which are common to both the +IN and –IN

The input current on the analog inputs depends upon a

number of factors: sample rate, input voltage, and source

impedance. Essentially, the current into the ADS8322 charges

the internal capacitor array during the sample period. After

this capacitance has been fully charged, there is no further

input current. The source of the analog input voltage must be

able to charge the input capacitance (25pF) to a 16-bit

settling level within the acquisition time (400ns) of the

device. When the converter goes into Hold mode, the input

impedance is greater than 1G

Ω.

Care must be taken regarding the absolute analog input

voltage. To maintain the linearity of the converter, the –IN

input should not drop below GND – 100mV or exceed

GND + 0.5V. The +IN input should always remain within

these ranges, the converter’s linearity may not meet specifi-

cations. To minimize noise, low-bandwidth input signals

with lowpass filters should be used.

DIGITAL INTERFACE

TIMING AND CONTROL

See the timing diagram in the Timing Characteristics section

for detailed information on timing signals and their require-

ment.

The ADS8322 uses an external clock (CLOCK) which

controls the conversion rate of the CDAC. With a 10MHz

external clock, the A/D converter sampling rate is 500kHz,

which corresponds to a 2

µs maximum throughput time.

Conversions are initiated by bringing the CONVST pin

LOW for a minimum of 20ns (after the 20ns minimum

requirement has been met, the CONVST pin can be brought

HIGH), while CS is LOW. The ADS8322 will switch from

Sample-to-Hold mode on the falling edge of the CONVST

command. Following the first rising edge of the external

clock after a CONVST LOW, the ADS8322 will begin

conversion (this first rising edge of the external clock repre-

sents the start of clock cycle one; the ADS8322 requires 16

rising clock edges to complete a conversion). The BUSY

output will go HIGH immediately following CONVST go-

ing LOW. BUSY will stay HIGH through the conversion

process and return LOW when the conversion has ended.

Both RD and CS can be HIGH during and before a conver-

sion (although CS must be LOW when CONVST goes LOW

to initiate a conversion). Both the RD and CS pins are

brought LOW in order to enable the parallel output bus with

the conversion.

READING DATA

The ADS8322 outputs full parallel data in Straight Binary

format, as shown in Table I. The parallel output will be

active when CS and RD are both LOW. The output data

should not be read 125ns prior to the falling edge of

CONVST and 10ns after the falling edge. Any other combi-

nation of CS and RD will tri-state the parallel output. Refer

to Table I for ideal output codes.

DESCRIPTION

ANALOG VALUE

Full-Scale Range

Least Significant

Bit (LSB)

BINARY CODE

HEX CODE

+Full Scale

1111 1111 1111 1111

FFFF

Midscale

1000 0000 0000 0000

8000

Midscale – 1LSB

0111 1111 1111 1111

7FFF

Zero

0V

0000 0000 0000 0000

0000

DIGITAL OUTPUT

STRAIGHT BINARY

TABLE I. Ideal Input Voltages and Output Codes.

BYTE

The output data will appear as a full 16-bit word on DB15-

DB0 (MSB-LSB), if BYTE is LOW. The result may also be

read on an 8-bit bus by using only DB7-DB0. In this case two

reads are necessary. The first, as before, leaving BYTE LOW

and reading the 8 least significant bits on DB7-DB0, then

bringing BYTE HIGH. When BYTE is HIGH, the upper 8 bits

(D15-D8) will appear on DB7-DB0.

NOISE

Figure 2 shows the transition noise of the ADS8322. A low-

level DC input was applied to the analog-input pins and the

converter was put through 8,192 conversions. The digital

output of the A/D converter will vary in output code due to

the internal noise of the ADS8322. This is true for all 16-bit

SAR-type A/D converters. Using a histogram to plot the

output codes, the distribution should appear bell-shaped

FIGURE 2. Histogram of 8,192 Conversions of a Low Level

DC Input.

5052

Code

0015

0014

0018

0017

0016

300

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818