9

®

PCM63P

for both if no immediate improvement were noted). This

procedure would require the generation of the digital bit-2

major carry code to the input of the PCM63P and a DVM or

oscilloscope capable of reading the output voltage for a one

LSB step (5.72

µV) in addition to a distortion analyzer.

A more practical approach would be to forego the minor

correction for the bit-2 major carry adjustment and only

adjust for upper and lower DAC gain matching. The prob-

lem is that just by connecting the MSB circuitry to the

PCM63P, the odds are that the upper and lower bit-2 weights

would be greatly changed from their unadjusted states and

thereby adversely affect the desired gain adjustment. Just

centering the 100k

Ω potentiometers would not necessarily

provide the correct starting point. To guarantee that each

100k

Ω potentiometer would be set to the correct starting or

null point (no current into or out of the MSB adjust pins), the

voltage drop across each corresponding 330k

Ω resistor would

have to measure 0V. A voltage drop of

±1.25mV across

either 330k

Ω resistor would correspond to a ±1LSB change

in the null point from its unadjusted state (1LSB in current

or 3.81nA x 330k

Ω = 1.26mV). Once these starting points

for each potentiometer had been set, each potentiometer

would then be adjusted equally, in opposite directions, to

achieve the lowest full-scale THD+N possible. If no imme-

diate improvement were noted, the direction of rotation for

both potentiometers would be reversed. One direction of

potentiometer counter-rotations would only make the gain

mismatch and resulting THD+N worse, while the opposite

would gradually improve and then worsen the THD+N after

passing through a no mismatch point. The determination of

the correct starting direction would be arbitrary. This proce-

dure still requires a good DVM in addition to a distortion

analyzer.

Each user will have to determine if a small improvement in

full-scale THD+N for their application is worth the expense

of performing a proper MSB adjustment.

APPLICATIONS

The most common application for the PCM63P is in high-

performance and professional digital audio playback, such

as in CD and DAT players. The circuit in Figure 6 shows the

PCM63P in a typical combination with a digital interface

format receiver chip (Yamaha YM3623), an 8x interpolating

digital filter (Burr-Brown DF1700P), and two third-order

low-pass anti-imaging filters (implemented using Burr-Brown

OPA2604APs).

Using an 8x digital filter increases the number of samples to

the DAC by a factor of 8, thereby reducing the need for a

higher order reconstruction or anti-imaging analog filter on

the DAC output. An analog filter can now be constructed

using a simple phase-linear GIC (generalized immittance

converter) architecture. Excellent sonic performance is

achieved using a digital filter in the design, while reducing

overall circuit complexity at the same time.

Because of its superior low-level performance, the PCM63P

is also ideally suited for other high-performance applications

such as direct digital synthesis (DDS).