Rainbow Electronics MAX1205 - Manual (Page 11)

Two’s Complement Output
The MAX1205 outputs data in two’s complement for-
mat. Table 1 shows how to convert the various full-
scale inputs into their two’s complement output codes.
Applications Information
Signal-to-Noise Ratio (SNR)
For a waveform perfectly reconstructed from digital
samples, the theoretical maximum SNR is the ratio of
full-scale analog input (RMS value) to the RMS quanti-
zation error (residual error). The ideal, theoretical mini-
mum analog-to-digital noise is caused by quantization
error only and results directly from the ADC’s resolution
(N bits):
SNR (MAX) = (6.02N + 1.76)dB
In reality, there are other noise sources besides quanti-
zation noise including thermal noise, reference noise,
clock jitter, etc. Therefore, SNR is computed by taking
the ratio of the RMS signal to the RMS noise, which
includes all spectral components minus the fundamen-
tal, the first nine harmonics, and the DC offset.
Signal-to-Noise Plus Distortion (SINAD)
SINAD is the ratio of the fundamental input frequency’s
RMS amplitude to all other ADC output signals:
SINAD (dB) = 20log [(Signal RMS / (Noise +
Distortion) RMS ]
Effective Number of Bits (ENOB)
ENOB indicates the global accuracy of an ADC at a
specific input frequency and sampling rate. An ideal
ADC’s error consists of quantization noise only. With an
input range equal to the full-scale range for the ADC,
the effective number of bits can be calculated as fol-
lows:
ENOB = (SINAD - 1.76) / 6.02
Total Harmonic Distortion (THD)
THD is the ratio of the RMS sum of the first nine har-
monics of the input signal to the fundamental itself. This
is expressed as:
where V 1 is the fundamental amplitude, and V
2
through
V 9 are the amplitudes of the 2nd through 9th order har-
monics.
Spurious-Free
Dynamic Range (SFDR)
SFDR is the ratio of RMS amplitude of the fundamental
(maximum signal component) to the RMS value of the
next largest spurious component, excluding DC offset.
Grounding and
Power-Supply Decoupling
Grounding and power-supply decoupling strongly influ-
ence the performance of the MAX1205. At 14-bit reso-
lution, unwanted digital crosstalk may couple through
the input, reference, power-supply, and ground con-
nections; this adversely affects the SNR or SFDR. In
addition, electromagnetic interference (EMI) can either
couple into or be generated by the MAX1205.
Therefore, grounding and power-supply decoupling
guidelines should be closely followed.
THD 20log
V V V V
V
2
2
3
2
4
2
9
2
=

 

















+ + + ⋅ ⋅ ⋅ +
1
MAX1205
+5V Single-Supply, 1Msps, 14-Bit
Self-Calibrating ADC
______________________________________________________________________________________ 11
0111....1111
TWO’S COMPLEMENT
0111....1111
ONE’S COMPLEMENT
1111....1111 +FSR - 1LSB
SCALE OFFSET BINARY
Table 1. Two’s Complement Conversion
0110....0000 0110....0000 1110....0000 +3/4FSR
0100....0000 0100....0000
0010....0000 0010....0000 1010....0000 +1/4FSR
1100....0000 +1/2FSR
0000....0000 0000....0000
— 1111....1111 — -0
1110....0000 1101....1111
1100....0000 1011....1111 0100....0000
1000....0000 +0
-1/2FSR
0110....0000 -1/4FSR
1010....0000 1001....1111 0010....0000 -3/4FSR
1000....0001 1000....0000
1000....0000 — 0000....0000 -FSR
0000....0001 -FSR + 1LSB