MAX1132/MAX1133
shutdown is long enough that the supply voltage or
ambient temperature may have changed.
Supplies, Layout, Grounding
and Bypassing
For best system performance, use separate analog and
digital ground planes. The two ground planes should
be tied together at the MAX1132/MAX1133. Use pins 3
and 14 as the primary AGND and DGND, respectively.
If the analog and digital supplies come from the same
source, isolate the digital supply from the analog with a
low value resistor (10Ω).
The MAX1132/MAX1133 are not sensitive to the order
of AV
DD
and DV
DD
sequencing. Either supply can be
present in the absence of the other. Do not apply an
external reference voltage until after both AV
DD
and
DV
DD
are present.
Be sure that digital return currents do not pass through
the analog ground. All return current paths must be
low-impedance. A 5mA current flowing through a PC
board ground trace impedance of only 0.05Ω creates
an error voltage of about 250µV, or about 2LSBs error
with a ±4V full-scale system. The board layout should
ensure as much as possible that digital and analog sig-
nal lines are kept separate. Do not run analog and digi-
tal lines parallel to one another. If you must cross one
with the other, do so at right angles.
The ADC is sensitive to high-frequency noise on the
AV
DD
power supply. Bypass this supply to the analog
ground plane with 0.1µF. If the main supply is not ade-
quately bypassed, add an additional 1µF or 10µF low-
ESR capacitor in parallel with the primary bypass
capacitor.
Transfer Function
Figures 10 and 11 show the MAX1132/MAX1133’s
transfer functions. In unipolar mode, the output data is
binary format and in bipolar mode it is two’s comple-
ment.
Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values
on an actual transfer function from a straight line. This
straight line can be either a best-straight-line fit or a line
drawn between the end points of the transfer function,
once offset and gain errors have been nullified. INL for
the MAX1132/MAX1133 is measured using the end-
point method.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step-width and the ideal value of 1LSB. A
DNL error specification of less than 1LSB guarantees
no missing codes and a monotonic transfer function.
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 falling
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 ADCs resolution (N bits):
SNR = (6.02
✕
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, which
includes all spectral components minus the fundamen-
tal, the first five harmonics, 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
✕
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 ADCs error consists of quanti-
zation noise only. With an input range equal to the full-
scale range of the ADC, calculate the effective number
of bits 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:
