AD766–Definition of Specifications

–4–

REV. A

TOTAL HARMONIC DISTORTION

Total Harmonic Distortion (THD) is defined as the ratio of the

square root of the sum of the squares of the values of the har-

monics to the value of the fundamental input frequency. It is ex-

pressed in percent (%) or decibels (dB).

THD is a measure of the magnitude and distribution of integral

linearity error and differential linearity error. The distribution of

these errors may be different, depending on the amplitude of the

output signal. Therefore, to be most useful, THD should be

specified for both large and small signal amplitudes.

SETTLING TIME

Settling Time is the time required for the output to reach and

remain within a specified error band about its final value, mea-

sured from the digital input transition. It is the primary measure

of dynamic performance.

BIPOLAR ZERO ERROR

Bipolar Zero Error or midscale error is the deviation of the ac-

tual analog output from the ideal output (0 V) when the 2s

complement input code representing half scale (all 0s) is loaded

in the input register.

DIFFERENTIAL LINEARITY ERROR

Differential Linearity Error is the measure of the variation in

analog value, normalized to full scale, associated with a 1 LSB

change in the digital input. Monotonic behavior requires that

the differential linearity error not exceed 1 LSB in the negative

direction.

MONOTONICITY

A D/A converter is monotonic if the output either increases or

remains constant as the digital input increases.

SIGNAL-TO-NOISE RATIO

SNR is defined as the ratio of the fundamental to the square

root of the sum of the squares for the values of all the nonfun-

damental, nonharmonic signals for a specified bandwidth. SNR

is tested at full-scale input. The AD766 specifies SNR for

20 kHz and 250 kHz bandwidths.

FUNCTIONAL DESCRIPTION

Serial input data is clocked into the AD766’s shift register by

the falling edge of CLK. Data is presumed to be in twos

complement format with MSB (i.e., the sign bit) clocked in first.

The shift register converts the most recently clocked-in 16 bits

to a parallel word. The falling edge of the latch enable (LE) sig-

nal causes the most recent parallel word to be transferred to the

internal DAC input latch. See Figure 2 for detailed serial port

timing requirements.

The contents of the DAC input latch cause the 16-bit DAC to

generate a corresponding current. This

±1 mA current is avail-

the summing junction pin, SJ (Pin 11) and connect the feedback

op amp is in the inverting configuration. Using the internal

3 k

Ω feedback resistor, this op amp will produce ±3 V outputs.

One advantage of external pins at each end of the feedback

resistor is that it allows the user to implement a single pole

active low-pass filter simply by adding a capacitor across these

pins (Pins 10 and 13). The circuit can best be understood

redrawn as shown in Figure 1.

Figure 1. Low-Pass Filter Using External Capacitor

The frequency response from this filter will be

=

−R

F

Ω (±20%).

Figure 2. AD766 Serial Input Timing