MAX1040/MAX1042/MAX1046/MAX1048
10-Bit, Multichannel ADCs/DACs with FIFO,
Temperature Sensing, and GPIO Ports
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Unipolar ADC Offset Error
For an ideal converter, the first transition occurs at 0.5
LSB, above zero. Offset error is the amount of deviation
between the measured first transition point and the
ideal first transition point.
Bipolar ADC Offset Error
While in bipolar mode, the ADC’s ideal midscale transi-
tion occurs at AGND -0.5 LSB. Bipolar offset error is the
measured deviation from this ideal value.
ADC Gain Error
Gain error is defined as the amount of deviation
between the ideal transfer function and the measured
transfer function, with the offset error removed and with
a full-scale analog input voltage applied to the ADC,
resulting in all ones at DOUT.
DAC Offset Error
DAC offset error is determined by loading a code of
all zeros into the DAC and measuring the analog
output voltage.
DAC Gain Error
DAC gain error is defined as the amount of deviation
between the ideal transfer function and the measured
transfer function, with the offset error removed, when
loading a code of all ones into the DAC.
Aperture Jitter
Aperture jitter (t
AJ
) is the sample-to-sample variation in
the time between the samples.
Aperture Delay
Aperture delay (t
AD
) is the time between the rising
edge of the sampling clock and the instant when an
actual sample is taken.
Signal-to-Noise Ratio
For a waveform perfectly reconstructed from digital sam-
ples, signal-to-noise ratio (SNR) is the ratio of full-scale
analog input (RMS value) to the RMS quantization error
(residual error). The ideal, theoretical minimum analog-
to-digital noise is caused by quantization error only and
results directly from the ADC’s resolution (N bits):
SNR = (6.02 x N + 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 calculated by taking
the ratio of the RMS signal to the RMS noise. RMS noise
includes all spectral components to the Nyquist fre-
quency excluding the fundamental, the first five har-
monics, and the DC offset.
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
)
Effective Number of Bits
Effective number of bits (ENOB) indicates the global
accuracy of an ADC at a specific input frequency and
sampling rate. An ideal ADC’s error consists of quanti-
zation noise only. With an input range equal to the full-
scale range of the ADC, calculate the ENOB as follows:
ENOB = (SINAD - 1.76) / 6.02
Total Harmonic Distortion
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 amplitude, and V
2
through
V
6
are the amplitudes of the first five harmonics.
Spurious-Free Dynamic Range
Spurious-free dynamic range (SFDR) is the ratio of RMS
amplitude of the fundamental (maximum signal compo-
nent) to the RMS value of the next largest distortion
component.
ADC Channel-to-Channel Crosstalk
Bias the ON channel to midscale. Apply a full-scale sine
wave test tone to all OFF channels. Perform an FFT on
the ON channel. ADC channel-to-channel crosstalk is
expressed in dB as the amplitude of the FFT spur at the
frequency associated with the OFF channel test tone.
Intermodulation Distortion (IMD)
IMD is the total power of the intermodulation products
relative to the total input power when two tones, f1 and
f2, are present at the inputs. The intermodulation prod-
ucts are (f1 ± f2), (2 x f1), (2 x f2), (2 x f1 ± f2), (2 x f2 ±
f1). The individual input tone levels are at -7dBFS.
Small-Signal Bandwidth
A small -20dBFS analog input signal is applied to an
ADC so the signal’s slew rate does not limit the ADC’s
performance. The input frequency is then swept up to
the point where the amplitude of the digitized conver-
sion result has decreased by -3dB. Note that the T/H
performance is usually the limiting factor for the small-
signal input bandwidth.
