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LTC2301/LTC2305

23015f

APPLICATIONS INFORMATION

frequency. Figure 4 shows a typical SINAD of 73.2dB with

a 14kHz sampling rate and a 1kHz input. A SNR of 73.3dB

can be achieved with the LTC2301/LTC2305.

Figure 4. 1kHz Sine Wave 8192 Point FFT Plot

Total Harmonic Distortion (THD)

Total Harmonic Distortion (THD) is the ratio of the RMS

sum of all harmonics of the input signal to the fundamental

itself. The out-of-band harmonics alias into the frequency

bandbetweenDCandhalfthesamplingfrequency(fSMPL/2).

THD is expressed as:

THD = 20log

+ V3

+ V4

2...+ V

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN aretheamplitudesofthesecond

through Nth harmonics.

Internal Reference

The LTC2301/LTC2305 have an on-chip, temperature

compensated bandgap reference that is factory trimmed

to 2.5V (Refer to Figure 5a). It is internally connected

to a reference ampliﬁer and is available at VREF (Pin 7).

VREF should be bypassed to GND with a 2.2μF tantalum

capacitor to minimize noise. An 8k resistor is in series

with the output so that it can be easily overdriven by an

external reference if more accuracy and/or lower drift are

required as shown in Figure 5b. The reference ampliﬁer

gains the VREF voltage by 1.638 to 4.096V at REFCOMP.

To compensate the reference ampliﬁer, bypass REFCOMP

with a 10μF ceramic or tantalum capacitor in parallel with

a 0.1μF ceramic capacitor for best noise performance. The

internal reference buffer can also be overdriven from 1V

to VDD with an external reference at REFCOMP as shown

in Figure 5c. To do so, VREF must be grounded to disable

the reference buffer. This will result in an input range of

0V to VREFCOMP in unipolar mode and ±0.5 • VREFCOMP in

bipolar mode.

REFERENCE

AMP

0.1μF

10μF

2.2μF

REFCOMP

GND

VREF

2.5V

4.096V

LTC2301

LTC2305

23015 F05a

BANDGAP

REFERENCE

Figure 5a. LTC2301/LTC2305 Reference Circuit

Figure 5b. Using the LT1790A-2.5 as an External Reference

Figure 5c. Overdriving REFCOMP Using the LT1790A-4.096

0.1μF

10μF

23015 F05b

LT1790A-2.5

VOUT

VIN

VREF

LTC2301

LTC2305

GND

REFCOMP

2.2μF

0.1μF

10μF

23015 F05c

LT1790A-4.096

VOUT

VIN

VREF

LTC2301

LTC2305

GND

REFCOMP

0.1μF

FREQUENCY (kHz)

–70

–60

–50

–40

–30

–20

–10

–90

–110

–130

–80

–100

–120

–140

1234

23015 F04

SNR = 73.2dB

SINAD = 73.1dB

THD = –80dB

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