LTC2238 データシートの表示（PDF） - Linear Technology

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LTC2238

10-Bit, 65/40/25Msps Low Noise 3V ADCs

Linear Technology 

LTC2238/LTC2237/LTC2236

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 1µF ceramic capacitor.

LTC2238/LTC2237/LTC2236

1.5V

VCM

4Ω

1.5V BANDGAP

REFERENCE

2.2µF

1V 0.5V

TIE TO VDD FOR 2V RANGE;

TIE TO VCM FOR 1V RANGE;

RANGE = 2 • VSENSE FOR

0.5V < VSENSE < 1V

1µF

SENSE

REFH

RANGE

DETECT

AND

CONTROL

BUFFER

INTERNAL ADC

HIGH REFERENCE

2.2µF

1µF

0.1µF

REFL

DIFF AMP

INTERNAL ADC

LOW REFERENCE

Figure 9. Equivalent Reference Circuit

1.5V

12k

0.75V

12k

VCM

2.2µF

SENSE

1µF

LTC2238

LTC2237

LTC2236

223876 F09

223876 F10

Figure 10. 1.5V Range ADC

18

223876fa

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