Rainbow Electronics MAX1208 - Manual (Page 24)

MAX1208
respectively. The feedback around the MAX4230 op
amps provides additional 10Hz lowpass filtering. The
2.157V and 1.141V reference voltages set the full-scale
analog input range to ±1.016V.
A common power source for all active components
removes any concern regarding power-supply sequenc-
ing when powering up or down.
Grounding, Bypassing, and
Board Layout
The MAX1208 requires high-speed board layout design
techniques. Refer to the MAX1211 evaluation kit data
sheet for a board layout reference. Locate all bypass
capacitors as close to the device as possible, prefer-
ably on the same side of the board as the ADC, using
surface-mount devices for minimum inductance.
Bypass V
DD
to GND with a 0.1µF ceramic capacitor in
parallel with a 2.2µF ceramic capacitor. Bypass OV
DD
to GND with a 0.1µF ceramic capacitor in parallel with a
2.2µF ceramic capacitor.
Multilayer boards with ample ground and power planes
produce the highest level of signal integrity. All MAX1208
GNDs and the exposed backside paddle must be con-
nected to the same ground plane. The MAX1208 relies on
the exposed backside paddle connection for a low-induc-
tance ground connection. Use multiple vias to connect
the top-side ground to the bottom-side ground. Isolate
the ground plane from any noisy digital system ground
planes such as a DSP or output buffer ground.
Route high-speed digital signal traces away from the
sensitive analog traces. Keep all signal lines short and
free of 90
° turns.
Ensure that the differential analog input network layout
is symmetric and that all parasitics are balanced equal-
ly. Refer to the MAX1211 evaluation kit data sheet for
an example of symmetric input layout.
Parameter Definitions
Integral Nonlinearity (INL)
Integral nonlinearity is the deviation of the values on an
actual transfer function from a straight line. For the
MAX1208, this straight line is between the end points of
the transfer function, once offset and gain errors have
been nullified. INL deviations are measured at every
step of the transfer function and the worst-case devia-
tion is reported in the Electrical Characteristics table.
Differential Nonlinearity (DNL)
Differential nonlinearity is the difference between an
actual step width and the ideal value of 1 LSB. A DNL
error specification of less than 1 LSB guarantees no
missing codes and a monotonic transfer function. For
the MAX1208, DNL deviations are measured at every
step of the transfer function and the worst-case devia-
tion is reported in the Electrical Characteristics table.
Offset Error
Offset error is a figure of merit that indicates how well
the actual transfer function matches the ideal transfer
function at a single point. Ideally the midscale
MAX1208 transition occurs at 0.5 LSB above midscale.
The offset error is the amount of deviation between the
measured midscale transition point and the ideal mid-
scale transition point.
Gain Error
Gain error is a figure of merit that indicates how well the
slope of the actual transfer function matches the slope of
the ideal transfer function. The slope of the actual trans-
fer function is measured between two data points: posi-
tive full scale and negative full scale. Ideally, the positive
full-scale MAX1208 transition occurs at 1.5 LSBs below
positive full scale, and the negative full-scale transition
occurs at 0.5 LSB above negative full scale. The gain
error is the difference of the measured transition points
minus the difference of the ideal transition points.
Small-Signal Noise Floor (SSNF)
Small-signal noise floor is the integrated noise and dis-
tortion power in the Nyquist band for small-signal
inputs. The DC offset is excluded from this noise calcu-
lation. For this converter, a small signal is defined as a
single tone with an amplitude less than -35dBFS. This
parameter captures the thermal and quantization noise
characteristics of the converter and can be used to
help calculate the overall noise figure of a receive
channel. Go to www.maxim-ic.com for application
notes on thermal + quantization noise floor.
Signal-to-Noise Ratio (SNR)
For a waveform perfectly reconstructed from digital sam-
ples, the theoretical maximum SNR is the ratio of the full-
scale analog input (RMS value) to the RMS quantization
error (residual error). The ideal, theoretical minimum ana-
log-to-digital noise is caused by quantization error only
and results directly from the ADC’s resolution (N bits):
SNR [max] = 6.02
× N + 1.76
12-Bit, 80Msps, 3.3V ADC
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