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ADS-325A

10-Bit, 20MHz Sampling A/D Converter

DATEL Data Acquisition products  

®

®

ADS-325A

Re-initiating the Start-up Calibration

The start-up calibration function can be re-initiated at any time

desired after the power and the references are supplied. Apply

a positive pulse to CE pin (pin 24) or a negative pulse to

RESET pin (pin 15). The pulse width of these pulses must be

equal to or wider than one A/D clock cycle. Also due to this

feature, you can make sure of a proper start-up calibration at

power-up by making a C-R delay connection with the RESET

pin as shown in Figure 4c.

Using Start-up Calibration Function Only

Internal and external calibration functions need not be

employed after start-up calibration. To use only the start-up

calibration function, connect the SEL pin (pin 17) to AGND and

the CAL pin (pin 14) to +AVS or AGND.

Auto Calibration Function

After the start up calibration is completed, the internal

calibration function can periodically and automatically generate

calibration pulses when the auto calibration mode is enabled.

To enable this function connect the SEL pin (pin 17) and the

CAL pin (pin 41) to +AVS. In this mode a 24-bit counter is

counted with every 16 A/D clock cycles and the carry-out is

used as the calibration pulse. The period of the calibration

pulse generated is as follows:

Period of Auto-calibration pulse = 1/fCLK x 16 x 16,777,216

For the case when the A/D clock frequency is 14.3MHz, the

calibration pulse generation cycle is 18.8 seconds. Since a

single calibration process is performed once every seven

pulses, the total calibration cycle is approximately

132 seconds.

A/D CLOCK

1/16

+AVS

14-BIT COUNTER

CO

CLR

D

Q

CLR

OUT

+AVS

AGND

VRT

VRB

RESET

CE

SENSE

AMP #1

SENSE

AMP #2

24-BIT COUNTER

CO

CLR

SEL

CAL

Figure 4a Internal Calibration Pulse Generation Circuit

VOLTS

+5

1V

+2.5

+AVS

VRT

VRB

0

TIME

SENSE AMP #1

SENSE AMP #2

CLR

RESET = HIGH, CE = LOW

Figure 4b. Conditions for Start-Up Calibration

+AVS

5V

15

+AVS

VRT

RESET

VRB

CE = "L"

Figure 4c. Start-up Calibration using RESET

T PW1

T PW0

CLOCK

+1.65V

Tsd

ANALOG

INPUT

N

TDL

OUTPUT

DATA

N+1

N–3

N+2

N–2

N+3

N–1

+1.65V (+DVS = +3.3V)

+2.5V (+DVS = +5.0V)

OUTPUT

N+4

ENABLE (OE)

OUTPUT

N

DATA

ACTIVE

TPEZ

TPZE

HIGH IMPEDENCE

+1.65V (+DVS = +3.3V)

+2.5V (+DVS = +5.0V)

= SAMPLING POINT

Figure 5. ADS-325A Timing Diagrams

+1.65V (+DVS = +3.3V)

+2.5V (+DVS = +5.0V)

ACTIVE

5

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