
LTC2435/LTC2435-1
30
24351fc
applicaTions inForMaTion
for each ohm of source resistance driving REF+ or REF–.
When FO = HIGH (internal oscillator and 50Hz notch), the
typical differential reference resistance is 18.7MΩ which
will generate a +FS gain error of approximately 0.027ppm
for each ohm of source resistance driving REF+ or REF–.
For the LTC2435-1, the typical differential reference resis-
tance is 17.1MΩ which will generate a +FS gain error of
approximately0.029ppmforeachohmofsourceresistance
driving REF+ or REF–. When FO is driven by an external
oscillatorwithafrequencyfEOSC(externalconversionclock
Figure 23. +FS Error vs RSOURCE at REF+ and REF– (Large CREF)
Figure 24. –FS Error vs RSOURCE at REF+ and REF– (Large CREF)
RSOURCE (Ω)
100
90
80
70
60
50
40
30
20
10
0
+FS
ERROR
VARIATION
(ppm)
2435 F23
0
400
800
1200
1600
2000
VCC = 5V
VREF+ = 5V
VREF– = GND
VIN+ = 3.75V
VIN– = 1.25V
FO = GND
TA = 25°C
CIN = 1μF, 10μF
CIN = 0.01μF
CIN = 0.1μF
RSOURCE (Ω)
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
–FS
ERROR
VARIATION
(ppm)
2435 F24
0
400
800
1200
1600
2000
VCC = 5V
VREF+ = 5V
VREF– = GND
VIN+ = 1.25V
VIN– = 3.75V
FO = GND
TA = 25°C
CIN = 1μF, 10μF
CIN = 0.01μF
CIN = 0.1μF
Figure 21. +FS Error vs RSOURCE at REF+ or REF– (Small CIN)
Figure 22. –FS Error vs RSOURCE at REF+ or REF– (Small CIN)
RSOURCE (Ω)
1
+FS
ERROR
VARIATION
(ppm)
100
90
80
70
60
50
40
30
20
10
0
–10
10000
2435 F21
10
100
1000
100000
CIN = 0pF
CIN = 0.01μF
VCC = 5V
VREF+ = 5V
VREF– = GND
VIN+ = 3.75V
VIN– = 1.25V
FO = GND
TA = 25°C
CIN = 100pF
CIN = 0.001μF
RSOURCE (Ω)
1
–FS
ERROR
VARIATION
(ppm)
10
0
–10
–20
–30
–40
–50
–60
–70
–80
–90
–100
10000
2435 F22
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 = 0.001μF
CIN = 100pF
operation),thetypicaldifferentialreferenceresistanceis2.4
1012/fEOSCΩ and each ohm of source resistance driving
REF+ or REF– will result in 0.21 10–6 fEOSCppm +FS
gain error. The effect of the source resistance on the two
reference pins is additive with respect to this gain error.
The typical +FS and –FS errors for various combinations
of source resistance seen by the REF+ and REF– pins and
external capacitance CREF connected to these pins are
shown in Figures 21, 22, 23 and 24.