MAX11156
18-Bit, 500ksps, ±5V SAR ADC
with Internal Reference in TDFN
Layout, Grounding, and Bypassing
For best performance, use PCBs with ground planes.
Ensure that digital and analog signal lines are separated
from each other. Do not run analog and digital lines paral-
lel to one another (especially clock lines), and avoid run-
ning digital lines underneath the ADC package. A single
solid GND plane configuration with digital signals routed
from one direction and analog signals from the other
provides the best performance. Connect the GND and
AGNDS pins on the MAX11156 to this ground plane. Keep
the ground return to the power-supply low impedance and
as short as possible for noise-free operation.
A 500pF C0G (or NPO) ceramic chip capacitor should
be placed between AIN+ and the ground plane as close
as possible to the MAX11156. This capacitor reduces the
inductance seen by the sampling circuitry and reduces
the voltage transient seen by the input source circuit.
For best performance, connect the REF output to the
ground plane with a 16V, 10FF ceramic chip capacitor
with a X5R or X7R dielectric in a 1210 or smaller case
size. Ensure that all bypass capacitors are connected
directly into the ground plane with an independent via.
Bypass VDD and OVDD to the ground plane with 0.1FF
ceramic chip capacitors on each pin as close as pos-
sible to the device to minimize parasitic inductance.
Add at least one bulk 10FF decoupling capacitor to VDD
and OVDD per PCB. For best performance, bring a
VDD power plane in on the analog interface side of the
MAX11156 and a OVDD power plane from the digital
interface side of the device.
Definitions
Integral Nonlinearity
Integral nonlinearity (INL) is the deviation of the values on
an actual transfer function from a straight line. For these
devices, this straight line is a line drawn between the end
points of the transfer function, once offset and gain errors
have been nullified.
Differential Nonlinearity
Differential nonlinearity (DNL) is the difference between
an actual step width and the ideal value of 1 LSB. For
these devices, the DNL of each digital output code is
measured and the worst-case value is reported in the
Electrical Characteristics table. A DNL error specification
of less than Q1 LSB guarantees no missing codes and a
monotonic transfer function.
Offset Error
For the MAX11156, the offset error is defined at the cen-
ter of code 0x2000. The center of code 0x2000 should
occur with an analog input voltage of exactly 0V. The
offset error is defined as the deviation between the actual
analog input voltage required to produce the center of
code 0x2000 and the ideal analog input of 0V, expressed
in LSBs.
Gain Error
Gain error is defined as the difference between the change
in analog input voltage required to produce a top code
transition minus a bottom code transition, subtracted from
the ideal change in analog input voltage on (5.0V/4.096V)
x VREF x (262142/262144). For the MAX11156, top code
transition is 0x3FFFE to 0x3FFFF. The bottom code tran-
sition is 0x00000 and 0x00001. For the MAX11156, the
analog input voltage to produce these code transitions is
measured and then the gain error is computed by subtract-
ing 2.0 x (5.0V/4.096V) x VREF x (262142/262144) from
this measurement.
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 power to the RMS quantization error
(residual error). The ideal, theoretical minimum analog-
to-digital noise is caused by quantization noise 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 quantiza-
tion noise: thermal noise, reference noise, clock jitter, etc.
SNR is computed by taking the ratio of the power signal to
the power noise, which includes all spectral components
not including the fundamental, 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 power to the power of all
the other ADC output signals:
SINAD ( dB=)
10 × log

Signal

(Noise + Distortion)
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