LTC2308 datasheet(13/20 Pages) LINER

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LTC2308

2308fb

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

band between DC and half the sampling frequency(fSMPL/2).

THD is expressed as:

THD

VVV

log

...

where V1 is the RMS amplitude of the fundamental fre-

quency and V2 through VN are the amplitudes of the second

through Nth harmonics.

Internal Reference

The LTC2308 has an on-chip, temperature compensated

bandgap reference that is factory trimmed to 2.5V (Refer

to Figure 6a). 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 6b. The reference ampliﬁer gains the

VREF voltage by 1.638 to 4.096V at REFCOMP (Pin 8). 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 AVDD with an external reference at REFCOMP as shown

in Figure 6c. 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.

Internal Conversion Clock

The internal conversion clock is factory trimmed to

achieve a typical conversion time (tCONV) of 1.3μs and a

maximum conversion time of 1.6μs over the full operat-

ing temperature range. With a typical acquisition time of

240ns, a throughput sampling rate of 500ksps is tested

and guaranteed.

APPLICATIONS INFORMATION

Figure 6a. LTC2308 Reference Circuit

Figure 6b. Using the LT1790A-2.5 as an External Reference

REFERENCE

AMP

0.1μF

10μF

2.2μF

REFCOMP

GND

VREF

2.5V

4.096V

LTC2308

2308 F06a

BANDGAP

REFERENCE

0.1μF

10μF

2308 F06b

LT1790A-2.5

VOUT

VIN

VREF

LTC2308

GND

REFCOMP

2.2μF

0.1 F

0.1μF

10μF

2308 F06c

LT1790A-4.096

VOUT

VIN

VREF

LTC2308

GND

REFCOMP

Figure 6c. Overdriving REFCOMP Using the LT1790A-4.096

Digital Interface

The LTC2308 communicates via a standard 4-wire SPI

compatible digital interface. The rising edge of CONVST

initiates a conversion. After the conversion is ﬁnished, pull

CONVST low to enable the serial output (SDO). The ADC

shifts out the digital data in 2’s complement format when

operating in bipolar mode or in straight binary format when

in unipolar mode, based on the setting of the UNI bit.

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