AD7769

–7–

REV. A

Plus and Minus Full-Scale Error

The ADC and DACs in the AD7769 can be considered as de-

vices with bipolar (plus and minus) input ranges, but referred to

difference between the actual input voltage at the FE to FF code

expressed in LSBs. Minus Full-Scale Error is similarly specified

for the 01 to 00 code transition, relative to the ideal input voltage

Error for the DACs is the difference, expressed in LSBs, between

the actual output voltage for input code FF and the ideal voltage

ADC and the DAC outputs are measured after their respective

Bias Offset errors have been adjusted out.

Digital-to-Analog Glitch Impulse

Digital-to-Analog Glitch Impulse is the impulse injected into the

analog outputs when the digital inputs change state with either

DAC selected. It is normally specified as the area of the glitch in

nV secs and is measured when the digital input code is changed

by 1 LSB at the major carry transition.

Digital Feedthrough

Digital Feedthrough is also a measure of the impulse injected

into the analog outputs from the digital inputs but is measured

when the DACs are not selected. This is essentially feedthrough

across the die and package. It is important in the AD7769 since

it is a measure of the glitch impulse transferred to the analog

outputs when data is read from the ADC register. It is specified

in nV secs and measured with

WR high and a digital code

change from all 0s to all 1s.

Signal-to-Noise Ratio (SNR)

SNR is the measured Signal-to-Noise Ratio at the output of the

converter. The signal is the rms magnitude of the fundamental.

Noise is the rms sum of all the nonfundamental signals up to

half the sampling frequency. SNR is dependent on the number

of quantization levels used in the digitization process; the more

levels, the smaller the quantization noise. The theoretical SNR

for a sine wave is given by

SNR = (6.02N + 1.76) dB

where N is the number of bits. Thus for an ideal 8-bit converter,

SNR = 49.92 dB.

Total Harmonic Distortion (THD)

THD is the ratio of the rms sum of harmonics to the fundamen-

tal. For the AD7769, Total Harmonic Distortion is defined as

20 log

2

+ V

3

2

+ V

4

2

+ V

5

2

+ V

6

1/ 2

V

1

harmonics.

Intermodulation Distortion (IMD)

products, of order (m+n), at sum and difference frequencies of

those for which neither m nor n is equal to zero. For example,

LOGIC TRUTH TABLE

ADC CHANNEL SELECT AND START CONVERSION

CS

ADC/DAC

CHA/CHB

WR

RD

DB0–DB7

INT

Comments

0

0

X

Note 1

Note 1

1

INT Is Set on Falling Edge of WR.

0

0

0

Note 1

Note 1

1

Select ADC Channel A and Start Conversion.

0

0

1

Note 1

Note 1

1

Select ADC Channel B and Start Conversion.

0

INT Goes Low at End of Conversion.

READ ADC DATA

CS

ADC/DAC

CHA/CHB

WR

RD

DB0–DB7

INT

Comments

0

X

X

X

ADC Data

1

INT Is Set High on Falling Edge of RD.

0

X

X

X

0

ADC Data

1

ADC Data on Data Bus.

0

X

X

X

High-Z

1

Data Outputs Impedance.

WRITE TO DACA OR DACB

CS

ADC/DAC

CHA/CHB

WR

RD

DB0–DB7

INT

Comments

01

0

1

µP Data

N/C

µP Writing Data to DACA.

01

1

1

µP Data

N/C

µP Writing Data to DACB.

0

1

0

0

ADC Data

N/C

Data from Last ADC Conversion Will Be Written to DACA.

0

1

1

0

ADC Data

N/C

Data from Last ADC Conversion Will Be Written to DACB.

1

X

X

X

X

High-Z

N/C

No Operation.

NOTES