LTC2421CMS datasheet(23/32 Pages) LINER

23 / 32 page

LTC2421/LTC2422

24212f

Input Current (VIN)

If complete settling occurs on the input, conversion re-

sults will be uneffected by the dynamic input current. If the

settling is incomplete, it does not degrade the linearity

performance of the device. It simply results in an offset/

full-scale shift, see Figure 17. To simplify the analysis of

input dynamic current, two separate cases are assumed:

large capacitance at VIN (CIN > 0.01µF) and small capaci-

tance at VIN (CIN < 0.01µF).

If the total capacitance at VIN (see Figure 18) is small

(< 0.01

µF), relatively large external source resistances (up

to 80k for 20pF parasitic capacitance) can be tolerated

without any offset/full-scale error. Figures 19 and 20 show

a family of offset and full-scale error curves for various

small valued input capacitors (CIN < 0.01µF) as a function

of input source resistance.

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 resis-

tance 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.

APPLICATIO S I FOR ATIO

Figure 18. An RC Network at VIN

Figure 19. Offset vs RSOURCE (Small C)

Figure 17. Offset/Full-Scale Shift

ZSSET

TUE

VIN

24212 F17

FSSET

CIN

24212 F17

INTPUT

SIGNAL

SOURCE

RSOURCE

VIN

LTC2421/

LTC2422

CPAR

≅20pF

RSOURCE (Ω)

10k

24212 F19

100

100k

VCC = 5V

VREF = 5V

VIN = 0V

TA = 25°C

CIN = 100pF

CIN = 1000pF

CIN = 0pF

CIN = 0.01µF

Figure 20. Offset vs RSOURCE (Large C)

Figure 21. Full-Scale Error vs RSOURCE (Large C)

RSOURCE (Ω)

600

800

24212 F20

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

24212 F21

–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

LTC2421CMS Datasheet(PDF) 23 Page - Linear Technology

LTC2421CMS Datasheet(HTML) 23 Page - Linear Technology

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32

Datasheet Descarga

Enlace URL