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ADC912A

CMOS Microprocessor-Compatible 12-Bit A/D Converter

Analog Devices 

ADC912A

INTERNAL CLOCK OSCILLATOR

4095

Figure 13 shows the ADC912A internal clock circuit. The clock

oscillates at the external crystal or ceramic resonator frequency.

4094

The 1.25 MHz crystal or ceramic resonator connects between

the CLK IN (Pin 17) and the CLK OUT (Pin 18). Capacitance

values (C1, C2) depend on the crystal or ceramic resonator

manufacturer. The crystal vendors should be qualified due to

2

variations in C1 and C2 values required from vendor to vendor.

Typical values range from 30 pF to 100 pF.

1

FULL-SCALE

TRANSITION

AT FS – 1.5 LSB

EXTERNAL CLOCK INPUT

A TTL compatible signal connected to CLK IN provides proper

converter clock operation. No connection is necessary to the

CLK OUT pin. The duty cycle of the external clock input can

vary from 45% to 55%. Figure 12 shows the important waveforms.

EXTERNAL REFERENCE

A low output resistance, negative five volt reference is necessary.

The external reference should be able to supply 3 mA of refer-

ence current. A bypass capacitor is necessary on the reference

input lead to minimize system noise as the internal DAC switches.

The reference input to the internal DAC is code dependent requir-

ing anywhere from zero to 3 mA. The reference voltage tolerance

has a direct influence on A/D converter full-scale voltage, and

the maximum input full-scale voltage equals 2 × –VREF. The

ADC912A is designed for ratiometric operation, but operation

using reference voltages between –5.00 V and 0 V will result in

degraded linearity performance. Integral linearity is fully tested and

guaranteed for references of –5 V. Figure 14 provides a good

–5 V reference that does not require precision resistors.

+5V TO +15V

2

INPUT

VOUT 6

REF02

TRIM 5

GND

4

0.01␮F

100⍀

10k⍀

2

V+

3 OP77

V–

+

100⍀

10␮F//0.01␮F

–5V

OUTPUT

–12V TO –15V

TRIM IS OPTIONAL, ONLY NECESSARY

FOR ABSOLUTE ACCURACY CIRCUITS

Figure 14. –5 V Reference

UNIPOLAR ANALOG INPUT OPERATION

Figure 15 shows the ideal input/output characteristic for the 0 V

to 10 V input range of the ADC912A. The designed output

code transitions occur midway between successive integer LSB

values (i.e., 0.5 LSB, 1.5 LSBs, 2.5 LSBs . . . FS – 1.5 LSBs).

The output code is natural binary with 1 LSB = FS/4096 =

(10/4096) V = 2.44 mV. The maximum full-scale input voltage

is (10 × 4095/4096) V = 9.9976 V.

0 0.5 1 2

FS-2 FS-1 FS

AIN – ANALOG INPUT IN LSB

Figure 15. Ideal ADC912A Input/Output Transfer

Characteristic

OFFSET AND FULL-SCALE ERROR ADJUSTMENT,

UNIPOLAR OPERATION

For applications where absolute accuracy is important, offset

and full-scale errors can be adjusted to zero. Figure 16 shows

the extra components required for full-scale error adjustment.

Zero offset is achieved by adjusting the null offset of the op amp

driving AIN.

VIN

0V TO 10V

+12V

37

6

10⍀

A1

2

5

4 10k⍀

1

ZERO

ADJUST

–12V

FULL

SCALE

ADJUST

200⍀

20k⍀

1 AIN

ADC912A*

3 AGND

A1: OP27 – LOWEST NOISE (TRIMMER CONNECTS

BETWEEN PINS 1 & 8, WIPER TO 12V)

OP42 – BEST BANDWIDTH

*EXTRA PINS OMITTED FOR CLARITY

Figure 16. Unipolar 0 V to 10 V Operation

Adjust the zero scale first by applying 1.22 mV (equivalent to

0.5 LSB input) to VIN. Adjust the op amp offset control until

the digital output toggles between 0000 0000 0000 and 0000

0000 0001. The next step is adjustment of full scale. Apply

9.9963 V (equivalent to FS – 1.5 LSB) to VIN and adjust R1

until the digital output toggles between 1111 1111 1110 and

1111 1111 1111.

REV. B

–9–

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