LTC2420 datasheet(24/36 Pages) LINER

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LTC2420

APPLICATIO S I FOR ATIO

For large input capacitor values (CIN > 0.01µF), the input

spikes are averaged by the capacitor into a DC current. The

gain shift becomes a linear function of input source

resistance independent of input capacitance, see Figures

21 and 22. The equivalent input impedance is 16.6M

Ω.

This results in

±150nA of input dynamic current at the

extreme values of VIN (VIN = 0V and VIN = VREF, when

VREF = 5V). This corresponds to a 0.3ppm shift in offset

and full-scale readings for every 10

Ω of input source

resistance.

In addition to the input current spikes, the input ESD

protection diodes have a temperature dependent leakage

current. This leakage current, nominally 1nA (

±10nA

max), results in a fixed offset shift of 10

µV for a 10k source

resistance.

RSOURCE (Ω)

10k

2420 F19

100

100k

VCC = 5V

VREF = 5V

VIN = 0V

TA = 25°C

CIN = 100pF

CIN = 1000pF

CIN = 0pF

CIN = 0.01µF

Figure 19. Offset vs RSOURCE (Small C)

Figure 20. Full-Scale Error vs RSOURCE (Small C)

Reference Current (VREF)

Similar to the analog input, the reference input has a

dynamic input current. This current has negligible effect

on the offset. However, the reference current at VIN = VREF

is similar to the input current at full-scale. For large values

of reference capacitance (CVREF > 0.01µF), the full-scale

error shift is 0.03ppm/

Ω of external reference resistance

independent of the capacitance at VREF, see Figure 23. If

the capacitance tied to VREF is small (CVREF < 0.01µF), an

input resistance of up to 80k (20pF parasitic capacitance

at VREF) may be tolerated, see Figure 24.

Unlike the analog input, the integral nonlinearity of the

device can be degraded with excessive external RC time

constants tied to the reference input. If the capacitance at

RSOURCE (Ω)

600

800

2420 F21

200

400

1000

CIN = 22µF

CIN = 10µF

CIN = 1µF

CIN = 0.1µF

CIN = 0.01µF

CIN = 0.001µF

VCC = 5V

VREF = 5V

VIN = 0V

TA = 25°C

RSOURCE (Ω)

–20

–15

–10

600

1000

2420 F22

–25

–30

–35

200

400

800

–5

CIN = 22µF

CIN = 10µF

CIN = 1µF

CIN = 0.1µF

CIN = 0.01µF

CIN = 0.001µF

VCC = 5V

VREF = 5V

VIN = 0V

TA = 25°C

Figure 21. Offset vs RSOURCE (Large C)

Figure 22. Full-Scale Error vs RSOURCE (Large C)

RSOURCE (Ω)

–50

–40

–30

–20

–10

100

10k

2420 F20

100k

VCC = 5V

VREF = 5V

VIN = 5V

TA = 25°C

CIN = 0.01µF

CIN = 100pF

CIN = 1000pF

CIN = 0pF

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