LTC2372-18_15 Datasheet, PDF (20/50 Pages) Linear Technology – 18-Bit, 500ksps, 8-Channel SAR ADC with 100dB SNR

LTC2372-18
Applications Information
Table 1 lists typical recommended values for the R and C
of each LPF mentioned.
Table 1. Recommended R and C Values for Each Lowpass Filter
Rx(Ω)
Cx(pF)
BANDWIDTH
LPF1
50
100000
31.8kHz
LPF2
10
1200
13MHz
LPF3
25
2700
2.4MHz
High quality capacitors and resistors should be used in the
RC filters since these components can add distortion. NPO
and silver mica type dielectric capacitors have excellent
linearity. Carbon surface mount resistors can generate
distortion from self heating and from damage that may
occur during soldering. Metal film surface mount resistors
are much less susceptible to both problems.
Input Currents
One of the biggest challenges in coupling an amplifier to
the LTC2372-18 is in dealing with current spikes drawn
by the MUX and ADC core analog inputs at the start of
each acquisition phase. LPF2 and LPF3 are examples of
coupling filters that are used to both filter noise and re-
duce sampling transients due to the current spikes.
The MUX and ADC core analog inputs may be modeled
as a switched capacitor load on the drive circuit. A drive
circuit may rely partially on attenuating switched-capacitor
current spikes with small filter capacitors C_FILT placed
directly at the ADC inputs and partially on the driver amplifier
having sufficient bandwidth to recover from the residual
disturbance. Amplifiers optimized for DC performance may
not have sufficient bandwidth to fully recover at the ADC’s
maximum conversion rate, which can produce nonlinearity
and other errors. Coupling filter circuits may be classified
in three broad categories:
Fully Settled: This case is characterized by filter time
constants and an overall settling time that are consider-
ably shorter than the sample period. When acquisition
begins, the coupling filter is disturbed. For a typical first
order RC filter, the disturbance will look like an initial step
with an exponential decay. The amplifier will have its own
response to the disturbance, which may include ringing. If
the input settles completely (to within the accuracy of the
LTC2372-18), the disturbance will not contribute any error.
20
LPF1
SIGNAL
SOURCES
LPF1
LPF1
LPF1
1/2 LPF1
CH0
LPF2 _CH1
CH2
LPF2 _CH3
CH4
LPF2 _CH5
CH6
LPF2 _CH7
COM
1/2 LPF2
LTC2372-18
+
18-BIT ADC CORE
–
BANDLIMITING
SIGNAL SOURCE
NOISE
R_X
BANDLIMITING
BUFFER NOISE
AND REDUCING
SAMPLING TRANSIENTS
MUXOUT^+/–
LPF3
ADCIN^+/–
LPFx
C_X
R_X
C_X
BANDLIMITING
BUFFER NOISE
AND REDUCING
SAMPLING TRANSIENTS
Figure 5. Input Signal Chain
237218 F05
For more information www.linear.com/LTC2372-18
237218f