Datasheet

change in input signal. The input amplifier must have a
high enough slew rate to complete the required output
voltage change before the beginning of the acquisition
time. Figure 10 shows an example of this circuit using
the MAX427.
Figures 11 and 12 show how the MAX1178/MAX1188
analog input current varies depending on whether the
chip is operating or powered down. The part is fully
powered down between conversions if the voltage at
R/C is set high during the second falling edge of CS.
The input current abruptly steps to the powered-up
value at the start of acquisition. This step in the input
current can disrupt the ADC input, depending on the
driving circuit’s output impedance at high frequencies. If
the driving circuit cannot fully settle by the end of acqui-
sition, the accuracy of the system can be compromised.
To avoid this situation, increase the acquisition time, use
a driving circuit that can settle within t
ACQ
, or leave the
MAX1178/MAX1188 powered up by setting the voltage
at R/C low during the second falling edge of CS.
Layout, Grounding, and Bypassing
For best performance, use printed circuit boards. Do not
run analog and digital lines parallel to each other, and do
not lay out digital signal paths underneath the ADC pack-
age. Use separate analog and digital ground planes with
only one point connecting the two ground systems (ana-
log and digital) as close to the device as possible.
Route digital signals far away from sensitive analog and
reference inputs. If digital lines must cross analog lines,
do so at right angles to minimize coupling digital noise
onto the analog lines. If the analog and digital sections
share the same supply, isolate the digital and analog
supply by connecting them with a low-value (10Ω)
resistor or ferrite bead.
The ADC is sensitive to high-frequency noise on the
AV
DD
supply. Bypass AV
DD
to AGND with a 0.1µF
capacitor in parallel with a 1µF to 10µF low-ESR capaci-
tor with the smallest capacitor closest to the device.
Keep capacitor leads short to minimize stray inductance.
MAX1178/MAX1188
16-Bit, 135ksps, Single-Supply ADCs with
Bipolar Analog Input Range
______________________________________________________________________________________ 11
MAX1178
MAX1188
MAX427
ANALOG
INPUT
AIN
REF
Figure 10. MAX1178/MAX1188 Fast-Settling Input Buffer
-1.5
-0.5
-1.0
0.5
0
1.0
1.5
-5.0 0-2.5 2.5 5.0
MAX1178
ANALOG INPUT CURRENT
vs. ANALOG INPUT VOLTAGE
ANALOG INPUT VOLTAGE (V)
ANALOG INPUT CURRENT (mA)
STANDBY
MODE
SHUTDOWN
MODE
Figure 11. MAX1178 Analog Input Current
-1.5
-0.5
-1.0
0.5
0
1.0
1.5
-10 0-5 5 10
MAX1188
ANALOG INPUT CURRENT
vs. ANALOG INPUT VOLTAGE
ANALOG INPUT VOLTAGE (V)
ANALOG INPUT CURRENT (mA)
STANDBY
MODE
SHUTDOWN
MODE
Figure 12. MAX1188 Analog Input Current