
LTC2451
15
2451fg
Figure 13. Measured INL vs Input Voltage,
CIN = 0, VCC = 5V, TA = 25°C
Figure 14. LTC2451 Input Signal Attenuation vs Frequency
either in sleep or I/O modes. Thus, if the time constant of
the input RC circuit
t = RS CIN, is of the same order of
magnitude or longer than the time periods between actual
conversions, then one can consider the input current to
be reduced correspondingly.
These considerations need to be balanced out by the input
signal bandwidth. The 3dB bandwidth ≈ 1/(2
pRSCIN).
Finally, if the recommended choice for CIN is unac-
ceptable for the user’s specific application, an alternate
strategy is to eliminate CIN and minimize CPAR and RS.
In practical terms, this configuration corresponds to a
low impedance sensor directly connected to the ADC
through minimum length traces. Actual applications
include current measurements through low value sense
resistors, temperature measurements, low impedance
voltage source monitoring, and so on. The resultant INL
versus VIN is shown in Figure 13. The measurements of
Figure 13 include a capacitor CPAR corresponding to a
minimum sized layout pad and a minimum width input
trace of about 1" length.
Signal Bandwidth and Noise Equivalent Input
Bandwidth
TheLTC2451includesasinc1typedigitalfilterwiththefirst
notch located at f0 = 60Hz. As such, the 3dB input signal
bandwidthis26.54Hz.ThecalculatedLTC2451inputsignal
attenuation versus frequency over a wide frequency range
is shown in Figure 14. The calculated LTC2451 input signal
attenuation with low frequencies is shown in Figure 15.
The converter noise level is about 1.4VRMS, and can be
modeled by a white noise source connected at the input
of a noise-free converter.
APPLICATIONS INFORMATION
INPUT
SIGNAL
A
TTENUA
TION
(dB)
–40
0
2451 F14
–60
–80
–20
–100
INPUT SIGNAL FREQUENCY (MHz)
0
1.00
1.25
1.50
2.5
5.0
7.5
INPUT VOLTAGE (V)
0
INL
(LSB)
8
6
4
2
0
–2
–4
–6
–8
4
2451 F13
1
2
3
5
3.5
0.5
1.5
2.5
4.5
RS = 1k
RS = 10k
RS = 0