5 Digital Codes

One final consideration in data acquisition and conversion systems is the digital coding of the data at the output of the A/D converter. Data is usually encoded in either binary or binary-coded-decimal (BCD) form.

Binary encoded data formats are most commonly employed for digital computer-oriented applications where the processing is normally performed in binary notation. BCD data encoding is usually required in applications where the data is fed to decimal end devices such as digital readouts and printers. The majority of applications require binary encoding.

The most commonly used binary codes in A/D converters are:

1. Unipolar Straight Binary (USB) — used for unipolar ana- log signal ranges, i.e., 0 to ±5 V, 0 to ±10 V, etc.
2. Bipolar Offset Binary (BOB) — used for bipolar analog signal ranges, i.e., ±5 V, ±10 V, etc.
3. Bipolar Two’s Complement (BTC) — used for bipolar analog signal ranges in many digital computer applications.

Two BCD codes, unipolar BCD and sign-magnitude BCD (SMD) are used in A/D converters. The definition of these codes is shown in Table 4 and Table 5.

Table 4. Definition of Binary Codes

DEFINITION	OUTPUT DIGITAL CODE	USB CODE	BOB CODE(2)
	MSB ...... LSB
+Full Scale	111....11Φ (1)	+VFSR –1/2LSB
Mid Scale	100....00Φ	+VFSR /2	Zero
–Full Scale	000....00Φ	+1/2LSB
One Least Significant Bit

(1) Φ is the transition value of the LSB.

(2) BTC Code–invert the MSB (sign bit) of the digital code—ranges same as BOB codes.

Table 5. Definition of Decimal Codes

DEFINITION	OUTPUT DIGITAL CODE (3 DIGITS)	DECIMAL VALUE
		BCD CODE	SMD CODE
Sign	MSD(1).............LSD
+ Full Scale	1 1001 1001 1001	999	+999
Zero	1 0000 0000 0000	000	+000
–Full Scale	0 1001 1001 1001	N/A	–999
One Least Significant Bit		(2)	(2)

(1) MSD = Most Significat Digit.

(2) n represents number of digits – —4 bits per digit.