MAX11158 Datasheet, PDF (14/26 Pages) Maxim Integrated Products – 500ksps Throughput Rates Without Pipeline

MAX11158
18-Bit, 500ksps, ±5V SAR ADC with
Internal Reference in µMAX
Although the MAX11158 is easy to drive, an amplifier
buffer is recommended if the source impedance is such
that when driving a switch capacitor of ~40pF a 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 2nF
capacitor is attached to the AIN+ pin. This capacitance
reduces the size of the transient at the start of the acquisi-
tion period, which will generate an input signal dependent
offset error in some buffers..
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/12, 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
0.5µs acquisition time a source impedance of less than
1kΩ 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.
These amplifier features help 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. In the
case of the MAX11158, settling in a 0.5µs duration
requires a RC filter bandwidth of approximately 4MHz.
With this bandwidth, it is preferable to use an amplifier
that will produce an output noise spectral density of
less than 4.5nV/√Hz, to ensure that the overall SNR is
not degraded significantly. It is recommended to insert
an external RC filter at the MAX11158 AIN+ input to
attenuate out-of-band input noise and preserve the
ADCs SNR. The effective RMS noise at the MAX11158
AIN+ input is 76FV, thus additional noise from a buffer
circuit should be significantly lower in order to achieve
the maximum SNR performance.
3) THD performance: The input buffer amplifier used should
have better THD performance than the MAX11158
to ensure the THD of the digitized signal is not
degraded.
Table 1 summarizes the operational amplifiers that are
compatible with the MAX11158. The MAX9632 has suf-
ficient bandwidth, low enough noise and distortion to sup-
port the full performance of the MAX11158. The MAX9633
is a dual amp and can support buffering for true pseudo-
differential sampling.
Table 1. List of Recommended ADC Driver Op Amps for MAX11158
AMPLIFIER
MAX9632
MAX9633
INPUT-NOISE
DENSITY
(nV/√Hz)
1
3
SMALL-SIGNAL
BANDWIDTH
(MHz)
55
27
SLEW RATE
(V/µs)
30
18
THD
(dB)
-128
-128
I_CC
(mA)
COMMENTS
3.9 Low noise, THD at 10kHz
3.5 Low noise, dual amp, THD at 10kHz
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