®

PCM1750

10

A Gain Change

Rotates

Transfer

Function

Offset Change

Shifts

Transfer

Function

1FFFF

H

1FFFE

H

00001

00000

H

3FFFF

H

20001

H

20000

H

* Gain drift (mostly due to reference drift) rotates the transfer function

around the bipolar zero code (00000

).

NOTE: As the power supply voltages change (mostly due to the +V

supply), the transfer function rotates around BPZ. See the power

supply rejection specification in the spec table.

0.00V

+2.749979

–20.98µV

–2.75

HEX

H

bit-1. The switches of the CDAC and the switches of the

TDAC operate concurrently with each other, that is, when a

decision is made to keep or reject bit-1, the same decision is

made for the correction voltage for bit-1. Even though the

ratio stability of the nichrome resistors used in the TDAC

may not be as good as the poly capacitors, it is inconsequen-

tial because the correction voltage of each bit has a limited

range of adjustment.

The DLE at the major carry (a code change from 111...111

to 000...000; in binary two’s complement coding) is typi-

cally

±1/2 LSB at the 16-bit level, which is sufficient to

provide 90dB SNR and –30dB low level distortion (–60dB

input). For applications requiring less DLE at the major

carry, a pin is provided for each channel to make an external

MSB adjustment.

DISCUSSION

OF SPECIFICATIONS

RESOLUTION AND DYNAMIC RANGE

The theoretical resolution of the PCM1750 is 18-bits. The

maximum possible number of output codes or counts at 18-

bits is 262,144 or 108dB (calculated by raising 2 to the 18th

power). The relative accuracy of any A/D converter, how-

ever, is more a function of it’s absolute linearity and signal-

to-noise ratio than how many bits of resolution it has. These

more pertinent specifications are described later in this

section.

Dynamic range, as it is usually defined for digital audio

converters, is the measure of THD+N at an effective input

signal level of –60dB referred to 0dB. For the PCM1750 this

value is typically 90dB and a minimum of 88dB (for audio

bandwidth = 20Hz to 24kHz, THD+N at –60db = –30 db typ,

commonly used as a theoretical measure of dynamic range,

but it does not take into account the effects of distortion and

noise at low signal levels.

ANALOG INPUT RANGE

The analog input range for the PCM1750 is a bipolar

±2.75V

(nominal). Table I gives the precise input/output and volt-

age/code relationships for the PCM1750. Figure 5 shows

these same relationships in a graphical format. It should be

noted that the computed voltage input levels represent center

values (the midpoint between code transitions). Output cod-

ing is in binary two’s complement.

DIGITAL OUTPUT

ANALOG INPUT

VOLTAGE INPUT

262144 LSBs

Full Scale Range

5.50000000V

1 LSB

Minimum Step Size

20.98083496

µV

+Full Scale

+2.74997902V

Bipolar Zero

0.00000000V

Bipolar Zero –1LSB

–0.00002098V

–Full Scale

–2.75000000V

TABLE I. Analog Input to Digital Output Relationships.

From Figure 5, the effects of offset and gain errors can be

visualized. These errors can change value in response to

changes in temperature and/or supply voltage. In addition,

gain error (or the full scale range, FSR) changes in direct

SAMPLE AND HOLD PARAMETERS

Aperture Delay and Uncertainty

Aperture delay is the time required to switch from the

SAMPLE to HOLD mode. This time is typically 10ns for the

PCM1750 and it is constant. Aperture uncertainty (jitter) is

the amount of uncertainty associated with the aperture delay.

Aperture uncertainty affects the overall accuracy of the

converter and is greatest at the maximum input frequency of

the converter. The formula for determining the maximum

SNR (signal-to-noise ratio) expressed in total number of

quantization levels. A 15-bit SNR, therefore, would be

= (2 X

closely with the rated dynamic accuracy of the PCM1750

where THD+N = –88dB max. This means the typical aper-

ture jitter of PCM1750 only becomes a factor when input

signals to it exceed 97kHz and/or an SNR greater than 15

bits is desired.

Input Bandwidth

The full power bandwidth of the PCM1750 is that input

frequency above which significant distortion is observed

(THD+N > 10-bits or –60dB for a full scale input signal). In

the data sheet, this number is specified as typically being

500kHz. In wideband operation (when no digital filter is

used) the additional full power bandwidth of the PCM1750

FIGURE 5. Analog Input to Digital Output Diagram.