LTC2451 datasheet(13/16 Pages) LINER

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LTC2451

2451fd

Figure 11. LTC2451 Input Drive Equivalent Circuit

2. The bandwidth from VSIG is reduced at the input pin.

This bandwidth reduction isolates the ADC from high

frequency signals, and as such provides simple anti-

aliasing and input noise reduction.

3. Switching transients generated by the ADC are attenu-

ated before they go back to the signal source.

4. A large CIN gives a better AC ground at the input pin,

helping reduce reﬂections back to the signal source.

5. Increasing RS protects the ADC by limiting the current

during an outside-the-rails fault condition.

There is a limit to how large RS • CIN should be for a given

application. Increasing RS beyond a given point increases

the voltage drop across RS due to the input current,

to the point that signiﬁcant measurement errors exist.

Additionally, for some applications, increasing the RS• CIN

product too much may unacceptably attenuate the signal

at frequencies of interest.

For most applications, it is desirable to implement CIN as

a high quality 0.1

μF ceramic capacitor and RS ≤ 1k. This

capacitor should be located as close as possible to the

input pin. Furthermore, the area encompassed by this circuit

path, as well as the path length, should be minimized.

Figure 12. Measured INL vs Input Voltage,

CIN = 0.1μF, VCC = 5V, TA = 25°C

In the case of a 2-wire sensor that is not remotely

grounded, it is desirable to split RS and place series

resistors in the ADC input line and in the sensor ground

return line, which should be tied to the ADC GND pin using

a star connection topology.

Figure 12 shows the measured LTC2451 INL versus the

input voltage as a function of RS value with an input ca-

pacitor CIN = 0.1μF.

In some cases, RS can be increased above these guide-

lines. The input current is negligible when the ADC is

either in sleep or I/O modes. Thus, if the time constant of

the input RC circuit

τ = RS • CIN, is of the same order of

magnitude or longer than the time periods between actual

conversions, then one can consider the input current to

be reduced correspondingly.

These considerations need to be balanced out by the input

signal bandwidth. The 3dB bandwidth ≈ 1/(2

πRSCIN).

Finally, if the recommended choice for CIN is unacceptable

for the user’s speciﬁc application, an alternate strategy is to

eliminate CIN and minimize CPAR and RS. In practical terms,

this conﬁguration corresponds to a low impedance sensor

directly connected to the ADC through minimum length

Figure 13. Measured INL vs Input Voltage,

CIN = 0, VCC = 5V, TA = 25°C

APPLICATIONS INFORMATION

INPUT VOLTAGE (V)

–4

2451 F12

–8

–12

–16

RS = 10k

RS = 1k

RS = 0

INPUT VOLTAGE (V)

–2

–4

–6

–8

2451 F13

123

3.5

0.5

1.5

2.5

4.5

RS = 1k

RS = 10k

RS = 0

VSIG

2451 F11

ILEAK

RSW

15k

(TYP)

ICONV

CIN

VCC

CEQ

0.35pF

(TYP)

CPAR

–

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