Datasheet

Data Sheet ADAS3022
Rev. B | Page 27 of 40
IN0+
COM–
IN0
IN1+
IN2+
IN3+
IN4+
IN5+
IN6+
IN7+
IN0+
IN1+
IN2+
IN3+
IN4+
IN5+
IN6+
IN7+
IN1
IN2
IN3
IN4
IN5
IN6
IN7
COM
IN0
IN1
IN2
IN3
IN4
IN5
IN6
IN7
COM
A—8 CHANNELS,
SINGLE-ENDED
B—8 CHANNELS,
COMMON REFERENCE
10516-014
IN2+
IN3+
IN4+
IN5+
IN0
IN1
IN2
IN3
IN4
IN5
IN6
IN7
COM
IN0
IN1
IN2
IN3
IN4
IN5
IN6
IN7
COM
IN0+ (–)
C—4 CHANNELS,
DIFFERENTIAL
D—COMBINATION
COM–
IN0– (+)
IN1+ (–)
IN1– (+)
IN0+ (–)
IN0– (+)
IN1+ (–)
IN1– (+)
IN2+ (–)
IN2– (+)
IN3+ (–)
IN3– (+)
Figure 61. Multiplexed Analog Input Configurations
Channel Sequencer
The ADAS3022 includes a channel sequencer that is useful for
scanning channels in a repeated fashion. Refer to the Channel
Sequencer Details section for more information.
Auxiliary Input Channel
The ADAS3022 includes an auxiliary input channel pair (AUX+
and AUX−) that bypasses the mux and PGIA stages, allowing direct
access to the SAR ADC core for applications where the additional
dedicated channel pair is required. As detailed previously, the
inputs are protected only from AVDD and AGND because the high
voltage supplies are used for the mux and PGIA stages but not the
lower voltage ADC core.
When the source impedance of the driving circuit is low, the
AUX± inputs can be driven directly. Large source impedances
significantly affect the ac performance, especially THD. The dc
performance parameters are less sensitive to the input impedance.
The maximum source impedance depends on the amount of
THD that can be tolerated. The THD degrades as a function of
the source impedance and the maximum input frequency.
Driver Amplifier Choice
For systems that cannot drive AUX± directly, a suitable op amp
buffer should be used to preserve the ADAS3022 performance.
The driver amplifier must meet the following requirements:
• The noise generated by the driver amplifier must be kept as
low as possible to preserve the SNR and the transition noise
performance of the ADAS3022. The noise from the
amplifier is filtered by the ADAS3022 analog input circuit
or by an external filter, if one is used. Because the typical
noise of the ADAS3022’s SAR ADC core is 35 µV rms (V
REF
=
4.096 V), the SNR degradation due to the amplifier is












+
=
−
22
)(
2
π
35
35
log20
N
3dB
LOSS
Nef
SNR
where:
f
−3dB
is the input bandwidth (8 MHz) of the ADAS3022’s SAR
ADC core expressed in megahertz or the cutoff frequency of
an input filter, if one is used.
N is the noise gain of the amplifier (for example, 1 in buffer
configuration).
e
N
is the equivalent input noise voltage of the op amp
expressed in nV/√Hz.
• For ac applications, the driver should have a THD performance
commensurate with the ADAS3022.
• The analog input circuit must settle a full-scale step onto
the capacitor array at a 16-bit level (0.0015%). In amplifier
data sheets, settling at 0.1% to 0.01% is more commonly
specified. This may differ significantly from the settling
time at a 16-bit level and should be verified prior to driver
selection.
Table 8. Recommended Driver Amplifiers
Amplifier
Typical Application
ADA4841-1, ADA4841-2 Very low noise, small, and low power
ADA4897-1, ADA4897-2 Very low noise, low and high frequencies
AD8655 5 V single supply, low noise
AD8021, AD8022 Very low noise and high frequency
OP184 Low power, low noise, and low frequency
AD8605, AD8615 5 V single supply, low power