
LTC2435/LTC2435-1
27
24351fc
applicaTions inForMaTion
Figure 15. An RC Network at IN+ and IN–
Figure 16. +FS Error vs RSOURCE at IN+ or IN– (Small CIN)
Figure 17. –FS Error vs RSOURCE at IN+ or IN– (Small CIN)
CIN
2435 F19
VINCM + 0.5VIN
RSOURCE
IN+
LTC2435/
LTC2435-1
CPAR
20pF
CIN
VINCM – 0.5VIN
RSOURCE
IN–
CPAR
20pF
RSOURCE (W)
1
+FS
ERROR
VARIATION
(ppm)
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
10000
2435 F16
10
100
1000
100000
CIN = 0pF
CIN = 100pF
CIN = 0.01μF
CIN = 0.001μF
VCC = 5V
VREF+ = 5V
VREF– = GND
VIN+ = 3.75V
VIN– = 1.25V
FO = GND
TA = 25°C
RSOURCE (W)
1
–FS
ERROR
VARIATION
(ppm)
100
90
80
70
60
50
40
30
20
10
0
–10
10000
2435 F17
10
100
1000
100000
CIN = 0pF
CIN = 0.01μF
VCC = 5V
VREF+ = 5V
VREF– = GND
VIN+ = 1.25V
VIN– = 3.75V
FO = GND
TA = 25°C
CIN = 100pF
CIN = 0.001μF
the sampling charge transfers when integrated over a
substantial time period (longer than 64 internal clock
cycles).
The effect of this input dynamic current can be analyzed
using the test circuit of Figure 15. The CPAR capacitor
includes the LTC2435/LTC2435-1 pin capacitance
(5pF typical) plus the capacitance of the test fixture
used to obtain the results shown in Figures 16 and
17. A careful implementation can bring the total input
capacitance (CIN + CPAR) closer to 5pF thus achieving
better performance than the one predicted by Figures
16 and 17. For simplicity, two distinct situations can
be considered.
For relatively small values of input capacitance (CIN <
0.01F), the voltage on the sampling capacitor settles
almostcompletelyandrelativelylargevaluesforthesource
impedance result in only small errors. Such values for CIN
will deteriorate the converter offset and gain performance
without significant benefits of signal filtering and the user
is advised to avoid them. Nevertheless, when small val-
ues of CIN are unavoidably present as parasitics of input
multiplexers, wires, connectors or sensors, the LTC2435/
LTC2435-1 can maintain their exceptional accuracy while
operating with relative large values of source resistance
as shown in Figures 16 and 17. These measured results
may be slightly different from the first order approxima-