LTC1289BCSW datasheet(18/28 Pages) LINER

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LTC1289

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APPLICATI

I FOR ATIO

settling can be extended by using a slower ACLK fre-

quency. At the maximum ACLK rate of 2MHz, RSOURCE– <

200

Ω and C2 < 20pF will provide adequate settling.

Input Op Amps

When driving the analog inputs with an op amp it is

important that the op amp settle within the allowed time

(see Figure 10). Again, the “+” and “–” input sampling

times can be extended as described above to accommo-

date slower op amps. For single supply low voltage

applications the LT1006, LT1013 and LT1014 can be

made to settle well even with the minimum settling win-

dows of 4

µs (“+” input) and 2µs (“–” input) which occur

at the maximum clock rates (ACLK = 2MHz and SCLK =

1MHz). Figures 11 and 12 show examples of adequate and

poor op amp settling. The LT1077, LT1078 or LT1079 can

be used here to reduce power consumption. Placing an RC

network at the output of the op amps will improve the

settling response and also reduce the broadband noise.

RC Input Filtering

It is possible to filter the inputs with an RC network as

shown in Figure 13. For large values of CF (e.g., 1µF), the

capacitive input switching currents are averaged into a net

DC current. Therefore, a filter should be chosen with a

small resistor and large capacitor to prevent DC drops

across the resistor. The magnitude of the DC current is

approximately IDC = 100pF × VIN/tCYC and is roughly

proportional to VIN. When running at the minimum cycle

time of 40

µs, the input current equals 6.3µA at VIN = 2.5V.

In this case, a filter resistor of 10

Ω will cause 0.1LSB of

full-scale error. If a larger filter resistor must be used,

errors can be eliminated by increasing the cycle time as

shown in the typical curve of Maximum Filter Resistor vs

Cycle Time.

Input Leakage Current

Input leakage currents can also create errors if the source

resistance gets too large. For instance, the maximum input

leakage specification of 1

µA (at 85°C) flowing through a

source resistance of 1k

Ω will cause a voltage drop of 1mV

or 1.6LSB with VREF = 2.5V. This error will be much

reduced at lower temperatures because leakage drops

rapidly (see typical curve of Input Channel Leakage Cur-

rent vs Temperature).

Noise Coupling Into Inputs

High source resistance input signals (>500

Ω) are more

sensitive to coupling from external sources. It is prefer-

able to use channels near the center of the package (i.e.,

CH2-CH7) for signals which have the highest output

resistance because they are essentially shielded by the

HORIZONTAL: 500ns/DIV

Figure 11. Adequate Settling of Op Amps Driving Analog Input

Figure 13. RC Input Filtering

RFILTER

VIN

CFILTER

LTC1289 AIF13

LTC1289

“+”

“–”

IIDC

HORIZONTAL: 20

µs/DIV

Figure 12. Poor Op Amp Settling Can Cause A/D Errors

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