AD1862

–5–

REV. A

TOTAL HARMONIC DISTORTION + NOISE

Total Harmonic Distortion plus Noise (THD+N) is defined as

the ratio of the square root of the sum of the squares of the val-

ues of the harmonics and noise to the value of the fundamental

input frequency. It is usually expressed in percent (%) or deci-

bels (dB).

D-RANGE DISTORTION

D-Range Distortion is the ratio of the signal amplitude to the

distortion plus noise at –60 dB. In this case, an A-Weight filter

is used. The value specified for D-Range performance is the ra-

tio measured plus 60 dB.

SETTLING TIME

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

remain within

±1/2 LSB about its final value, measured from

the digital input transition. It is a primary measure of dynamic

performance and is usually expressed in nanoseconds (ns).

SIGNAL-TO-NOISE RATIO

The Signal-to-Noise Ratio is defined as the ratio of the ampli-

tude of the output with full-scale present to the amplitude of the

output when no signal is present. It is expressed in decibels (dB)

and measured using an A-Weight filter.

GAIN LINEARITY

Gain Linearity is a measure of the deviation of the actual output

amplitude from the ideal output amplitude. It is determined by

measuring the amplitude of the output signal as the amplitude

of that output signal is digitally reduced to a low level. A perfect

D/A converter exhibits no difference between the ideal and ac-

tual amplitudes. Gain linearity is expressed in decibels (dB).

MIDSCALE ERROR

Midscale Error, or bipolar zero error, is the deviation of the ac-

tual analog output from the ideal output when the 2s comple-

ment input code representing midscale is loaded in the input

register. The AD1862 is a current output D/A converter. There-

fore, this error is expressed in

µA.

DECODER AND

DIGITAL OFFSET

20-BIT DAC

LATCH

SERIAL INPUT

REGISTER

TRIM

ADJ

FEEDBACK

REGISTER

CURRENT

OUTPUT

V

L

+

NR1

V

S

AGND

V

REF

NR2

– V

L

LATCH

ENABLE

CLOCK

DATA

DGND

AD1862 Block Diagram

FUNCTIONAL DESCRIPTION

The AD1862 is a high performance, monolithic 20-bit audio

DAC. Each device includes a voltage reference, a 20-bit DAC,

20-bit input latch and a 20-bit serial-to-parallel input register. A

special digital offset circuit, combined with segmentation cir-

cuitry, produces excellent THD+N and D-range performance.

Extensive noise-reduction features are utilized to make the noise

performance of the AD1862 as high as possible. For example,

the voltage reference circuit is a low-noise, 9 volt bandgap cell.

This cell supplies the reference voltage to the bipolar offset cir-

cuit and the DAC. An external noise-reduction capacitor is con-

nected to NR1 to form a low-pass filter network.

Additional noise-reduction techniques are used in the control

amplifier of the DAC. By connecting an external noise-reduction

capacitor to NR2 output noise contributions from the control

portion of the DAC are similarly reduced. The noise-reduction

efforts result in a signal-to-noise ratio of 120 dB.

The design of the AD1862 uses a combination of segmented de-

coder, R-2R topology and digital offset to produce low distor-

tion at all signal amplitudes. The digital offset technique shifts

the midscale output voltage (0 V) away from the MSB transition

of the device. Therefore, small amplitude signals are not af-

fected by an MSB change. An extra DAC cell is included to

avoid clipping the output at full scale.

The DAC supplies a

±1 mA output current to an external

I-to-V converter. An on-board 3 k

Ω feedback resistor is also

supplied. Both the output current and feedback resistor are

laser-trimmed to

±2% tolerance, simplifying the selection of

external filter and/or deemphasis network components. The in-

put register and serial-to-parallel converter are fabricated with

CMOS logic gates. These gates allow the achievement of fast

switching speeds and low power consumption. Internal TTL-

to-CMOS converters are used to insure TTL and 5 V CMOS

compatibility.

OBSOLETE