LTC2053_1 datasheet(10/16 Pages) LINER

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LTC2053/LTC2053-SYNC

2053syncfb

APPLICATIO S I FOR ATIO

charging current decays exponentially during each input

sampling period with a time constant equal to RSCS. If the

voltage disturbance due to these currents settles before

the end of the sampling period, there will be no errors

due to source resistance or the source resistance mis-

match between –IN and +IN. With RS less than 10k, no

DC errors occur due to this input current.

In the Typical Performance Characteristics section of this

data sheet, there are curves showing the additional error

from non-zero source resistance in the inputs. If there are

no large capacitors across the inputs, the amplifier is less

sensitive to source resistance and source resistance mis-

match. When large capacitors are placed across the in-

puts, the input charging currents described above result in

larger DC errors, especially with source resistor mis-

matches.

Power Supply Bypassing

The LTC2053 uses a sampled data technique and therefore

contains some clocked digital circuitry. It is therefore

sensistive to supply bypassing. For single or dual supply

operation, a 0.1µF ceramic capacitor must be connected

between Pin 8 (V+) and Pin 4 (V–) with leads as short as

possible.

–

V+IN

VOUT

V–IN

–

V+IN

VOUT

V–IN

VREF

–5V

2053 F01

0V < V+IN < 5V

0V < V–IN < 5V

0V < VD < 3.7V

VOUT = VD

–5V < V–IN < 5V AND ⏐V–IN – VREF⏐ < 5.5V

–5V < V+IN < 5V AND ⏐V+IN – VREF⏐ < 5.5V

–5V < VD + VREF < 3.7V

SINGLE SUPPLY, UNITY GAIN

DUAL SUPPLY

6 R2

VOUT = 1 +

VD + VREF

()

Settling Time

The sampling rate is 3kHz and the input sampling period

during which CS is charged to the input differential voltage

VIN is approximately 150µs. First assume that on each

input sampling period, CS is charged fully to VIN. Since CS

= CH (= 1000pF), a change in the input will settle to N bits

of accuracy at the op amp noninverting input after N clock

cycles or 333µs(N). The settling time at the OUT pin is also

affected by the settling of the internal op amp. Since the

gain bandwidth of the internal op amp is typically 200kHz,

the settling time is dominated by the switched capacitor

front end for gains below 100 (see Typical Performance

Characteristics).

Input Current

Whenever the differential input VIN changes, CH must be

charged up to the new input voltage via CS. This results in

an input charging current during each input sampling

period. Eventually, CH and CS will reach VIN and, ideally,

the input current would go to zero for DC inputs.

In reality, there are additional parasitic capacitors which

disturb the charge on CS every cycle even if VIN is a DC

voltage. For example, the parasitic bottom plate capacitor

on CS must be charged from the voltage on the REF pin to

the voltage on the –IN pin every cycle. The resulting input

Figure 1

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