MAX1253/MAX1254
Stand-Alone, 10-Channel, 12-Bit System Monitors
with Internal Temperature Sensor and V
DD
Monitor
28 ______________________________________________________________________________________
Gain Error
The gain error is the difference between the ideal and
actual value of the analog input difference between the
first and last transitions of the ADC output. The first
transition is from digital code 0 to code 1, and the last
from code (2
N-2
) to code (2
N-1
), where N = number of
ADC bits for straight binary output code. For the
MAX1253/MAX1254, the ideal difference in the input
voltage between code transitions 0 to 1 and code tran-
sitions 4094 to 4095 is 4094 x LSB. For the MAX1253,
this is 2.5V - 2 x LSB = 2.498780V, and for the
MAX1254, this is 4.096V - 2 x LSB = 4.094V. Gain error
is a DC specification, usually normalized to the FS ideal
analog value and given in percent of FSR or ppm.
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital
samples, signal-to-noise ratio (SNR) is the ratio of the
full-scale analog input (RMS value) to the RMS quantiza-
tion error (residual error). The ideal theoretical minimum
analog-to-digital noise is caused by quantization error
only, results directly from the ADC’s resolution (N bits),
and can be calculated with the following equation:
SNR = (6.02 x N + 1.76)dB
Signal-to-Noise Plus Distortion
Signal-to-noise plus distortion (SINAD) is the ratio of the
fundamental input frequency’s RMS amplitude to the
RMS equivalent of all other ADC output signals:
SINAD (dB) = 20 x log (Signal
RMS
/ Noise
RMS
)
There are other noise sources besides quantization
noise, including thermal noise, reference noise, clock
jitter, etc. Therefore, SINAD is calculated by taking the
ratio of the full-scale signal to the RMS noise, which
includes all spectral components minus the fundamen-
tal and the first five harmonics.
Total Harmonic Distortion (THD)
Total harmonic distortion (THD) is the ratio of the RMS
sum of the first five harmonics of the input signal to the
fundamental itself. This is expressed as:
where V
1
is the fundamental RMS value, and V
2
through V
5
are the RMS values of the 2nd- through 5th-
order harmonics, respectively.
Power-Supply Rejection
Power-supply rejection is the ratio between the change
in the ADC full-scale output to the change in power-
supply voltage when the power-supply voltage is varied
from its nominal value. It is specified in V/V or µV/V.
