LTC2487
24
2487fd
Figure 14. +FS Error vs RSOURCE
at VREF (Large CREF)
Figure 15. –FS Error vs RSOURCE
at VREF (Large CREF)
RSOURCE (Ω)
0
+FS
ERROR
(ppm) 300
400
500
800
2487 F14
200
100
0
200
400
600
1000
VCC = 5V
VREF = 5V
VIN
+ = 3.75V
VIN
– = 1.25V
fO = GND
TA = 25°C
CREF = 1μF, 10μF
CREF = 0.1μF
CREF = 0.01μF
RSOURCE (Ω)
0
–FS
ERROR
(ppm) –200
–100
0
800
2487 F15
–300
–400
–500
200
400
600
1000
VCC = 5V
VREF = 5V
VIN
+ = 1.25V
VIN
– = 3.75V
fO = GND
TA = 25°C
CREF = 1μF, 10μF
CREF = 0.1μF
CREF = 0.01μF
For relatively small values of external reference capacitance
(CREF < 1nF), the voltage on the sampling capacitor settles
for reference impedances of many kΩ (if CREF = 100pF up
to 10kΩ will not degrade the performance (see Figures
12 and 13)
).
In cases where large bypass capacitors are required on
the reference inputs (CREF > 0.01μF), full-scale and linear-
ity errors are proportional to the value of the reference
resistance. Every ohm of reference resistance produces
a full-scale error of approximately 0.5ppm
(while operat-
ing in simultaneous 50Hz/60Hz mode (see Figures 14
and 15)
). If the input common mode voltage is equal to
the reference common mode voltage, a linearity error of
approximately 0.67ppm per 100Ω of reference resistance
results (see Figure 16). In applications where the input
and reference common mode voltages are different, the
errors increase. A 1V difference in between common mode
input and common mode reference results in a 6.7ppm
INL error for every 100Ω of reference resistance.
In addition to the reference sampling charge, the reference
ESD protection diodes have a temperature dependent leak-
age current. This leakage current, nominally 1nA (±10nA
max) results in a small gain error. A 100Ω reference
resistance will create a 0.5μV full-scale error.
Figure 12. +FS Error vs RSOURCE at VREF (Small CREF)
Figure 13. –FS Error vs RSOURCE at VREF (Small CREF)
APPLICATIONS INFORMATION
RSOURCE (Ω)
0
+FS
ERROR
(ppm)
50
70
90
10k
2487 F12
30
10
40
60
80
20
0
–10
10
100
1k
100k
VCC = 5V
VREF = 5V
VIN
+ = 3.75V
VIN
– = 1.25V
fO = GND
TA = 25°C
CREF = 0.01μF
CREF = 0.001μF
CREF = 100pF
CREF = 0pF
RSOURCE (Ω)
0
–FS
ERROR
(ppm) –30
–10
10
10k
2487 F13
–50
–70
–40
–20
0
–60
–80
–90
10
100
1k
100k
VCC = 5V
VREF = 5V
VIN
+ = 1.25V
VIN
– = 3.75V
fO = GND
TA = 25°C
CREF = 0.01μF
CREF = 0.001μF
CREF = 100pF
CREF = 0pF