MAX11156 Datasheet, PDF (13/27 Pages) Maxim Integrated Products – 18-Bit, 500ksps, ±5V SAR ADC with Internal Reference in TDFN

MAX11156
18-Bit, 500ksps, ±5V SAR ADC
with Internal Reference in TDFN
Reference Mode 11: The internal bandgap reference
source as well as the internal reference buffer are both
in a shutdown state. The REF pin is in a high-impedance
state. This mode would typically be used when an exter-
nal reference source and external reference buffer is used
to drive all MAX11156 parts in a system.
Regardless of the reference mode used, the MAX11156
requires a low-impedance reference source on the REF
pin to support 18-bit accuracy. When using the internal
reference buffer, externally bypass the reference buffer
output using at least a 10FF, low-inductance, low-ESR
capacitor placed as close as possible to the REF pin, thus
minimizing additional PCB inductance. When using the
internal bandgap reference source, bypass the REFIO pin
with a 0.1FF capacitor to ground. If providing an external
reference and using the internal reference buffer, drive
the REFIO pin directly with an external reference source
in the range of 3.0V to 4.25V. Finally, if disabling the
MAX11156 internal bandgap reference source and inter-
nal reference buffer, drive the REF pin with a reference
voltage in the range of 2.5V to 4.25V and place at least a
10FF, low-inductance, low-ESR capacitor placed as close
as possible to the REF pin .
When using the MAX11156 in external reference mode,
it is recommended that an external reference buffer be
used. For bypass capacitors on the REF pin, X7R or X5R
ceramic capacitors in a 1210 case size or smaller have
been found to provide adequate bypass performance.
Y5U or Z5U ceramic capacitors are not recommended
due to their high voltage and temperature coefficients.
Maxim offers a wide range of precision references ideal
for 18-bit accuracy. Table 1 lists some of the options rec-
ommended.
Input Amplifier
The conversion results are accurate when the ADC
acquires the input signal for an interval longer than
the input signal's settling time. The ADC input sampling
capacitor charges during the acquisition period. During
this acquisition period, the settling of the sampled voltage
is affected by the source resistance and the input sam-
pling capacitance. Sampling error can be estimated by
modeling the time constant of the total input capacitance
and the driving source impedance.
Although the MAX11156 is easy to drive, an amplifier buf-
fer is recommended if the source impedance is such that
when driving a switch capacitor of ~20pF a significant
settling error in the desired sampling period will occur. If
this is the case, it is recommended that a configuration
shown in the Typical Operating Circuit is used where at
least a 500pF capacitor is attached to the AIN+ pin. This
capacitance reduces the size of the transient at the start
of the acquisition period, which in some buffers will cause
an input signal dependent offsets.
Regardless of whether an external buffer amp is used or
not, the time constant, R_SOURCE × C_LOAD, of the input
should not exceed t_ACQ/13, where R_SOURCE is the total
signal source impedance, C_LOAD is the total capacitance
at the ADC input (external and internal) and t_ACQ is the
acquisition period. Thus to obtain accurate sampling in a
500ns acquisition time a source impedance of less than
1042Ω should be used if driving the ADC directly. When
driving the ADC from a buffer, it is recommended a series
resistance (5Ω to 50Ω typical) between the amplifier and
the external input capacitance as shown in the Typical
Operating Circuit.
We report some amplifier features to select the ADC
driver.
1) Fast settling time: For multichannel multiplexed appli-
cations the driving operational amplifier must be able
to settle to 18-bit resolution when a full-scale step is
applied during the minimum acquisition time.
2) Low noise: It is important to ensure that the driver
amplifier has a low average noise density appropriate
for the desired bandwidth of the application. When the
MAX11156 is used with its full bandwidth of 6MHz, it
Table 1. MAX11156 External Reference Recommendations
PART
MAX6126
V_OUT (V)
2.5, 3, 4.096, 5.0
TEMPERATURE
COEFFICIENT (MAX)
3 (A), 5 (B)
INITIAL
ACCURACY (%)
0.06
NOISE (0.1Hz TO
10Hz) (µV_P-P)
1.35
MAX6325
2.5
1
MAX6341
4.096
1
0.04
1.5
0.02
2.4
PACKAGE
µMAX-8
SO-8
SO-8
SO-8
www.maximintegrated.com
Maxim Integrated │  13