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LTC2238/LTC2237/LTC2236

223876fa

VCM

REFH

SENSE

TIE TO VDD FOR 2V RANGE;

TIE TO VCM FOR 1V RANGE;

RANGE = 2 • VSENSE FOR

0.5V < VSENSE < 1V

1.5V

REFL

2.2

µF

2.2

µF

INTERNAL ADC

HIGH REFERENCE

BUFFER

0.1

µF

223876 F09

Ω

DIFF AMP

µF

INTERNAL ADC

LOW REFERENCE

1.5V BANDGAP

REFERENCE

0.5V

RANGE

DETECT

AND

CONTROL

LTC2238/LTC2237/LTC2236

Figure 9. Equivalent Reference Circuit

APPLICATIO S I FOR ATIO

Reference Operation

Figure 9 shows the LTC2238/LTC2237/LTC2236 reference

circuitry consisting of a 1.5V bandgap reference, a differ-

ence amplifier and switching and control circuit. The inter-

nal voltage reference can be configured for two pin select-

able input ranges of 2V (

±1V differential) or 1V (±0.5V dif-

ferential). Tying the SENSE pin to VDD selects the 2V range;

tying the SENSE pin to VCM selects the 1V range.

The 1.5V bandgap reference serves two functions: its output

provides a DC bias point for setting the common mode

voltage of any external input circuitry; additionally, the

reference is used with a difference amplifier to generate the

differential reference levels needed by the internal ADC

circuitry. An external bypass capacitor is required for the

1.5V reference output, VCM. This provides a high frequency

low impedance path to ground for internal and external

circuitry.

The difference amplifier generates the high and low refer-

ence for the ADC. High speed switching circuits are

connected to these outputs and they must be externally

bypassed. Each output has two pins. The multiple output

pins are needed to reduce package inductance. Bypass

capacitors must be connected as shown in Figure 9.

Other voltage ranges in-between the pin selectable ranges

can be programmed with two external resistors as shown

in Figure 10. An external reference can be used by applying

its output directly or through a resistor divider to SENSE.

It is not recommended to drive the SENSE pin with a logic

device. The SENSE pin should be tied to the appropriate

level as close to the converter as possible. If the SENSE pin

is driven externally, it should be bypassed to ground as

close to the device as possible with a 1

µF ceramic capacitor.

Figure 10. 1.5V Range ADC

VCM

SENSE

1.5V

0.75V

2.2

µF

12k

µF

12k

223876 F10

LTC2238

LTC2237

LTC2236

LTC2238 Datasheet(PDF) 18 Page - Linear Technology

LTC2238 Datasheet(HTML) 18 Page - Linear Technology

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