LTC1609ACSW datasheet(13/24 Pages) LINER

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LTC1609

1609fa

APPLICATIO S I FOR ATIO

Dynamic Performance

FFT (Fast Fourier Transform) test techniques are used to

test the ADC’s frequency response, distortion and noise

at the rated throughput. By applying a low distortion sine

wave and analyzing the digital output using an FFT

algorithm, the ADC’s spectral content can be examined

for frequencies outside the fundamental. Figure 6 shows

a typical LTC1609 FFT plot which yields a SINAD of

87.2dB and THD of – 100dB.

Signal-to-Noise Ratio

The Signal-to-Noise and Distortion Ratio (SINAD) is the

ratio between the RMS amplitude of the fundamental input

frequency to the RMS amplitude of all other frequency

components at the A/D output. The output is band limited

to frequencies from above DC and below half the sampling

frequency. Figure 6 shows a typical SINAD of 87.2dB with

a 200kHz sampling rate and a 1kHz input.

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 Voltage Reference

The LTC1609 has an on-chip, temperature compensated,

curvature corrected, bandgap reference, which is factory

trimmed to 2.50V. The full-scale range of the ADC scales

with VREF. The output of the reference is connected to the

input of a unity-gain buffer through a 4k resistor (see

Figure 7). The input to the buffer or the output of the

reference is available at REF. The internal reference can be

overdriven with an external reference if more accuracy is

needed. The buffer output drives the internal DAC and is

available at CAP. The CAP pin can be used to drive a steady

DC load of less than 2mA. Driving an AC load is not

recommended because it can cause the performance of

the converter to degrade.

For minimum code transition noise the REF pin and the

CAP pin should each be decoupled with a capacitor to

filter wideband noise from the reference and the buffer

(2.2

µF tantalum).

Figure 6. LTC1609 Nonaveraged 4096 Point FFT Plot

–

1609 F07

INTERNAL

CAPACITOR

DAC

BANDGAP

REFERENCE

VANA

2.2

µF

CAP

(2.5V)

2.2

µF

REF

(2.5V)

Figure 7. Internal or External Reference Source

FREQUENCY (kHz)

–60

–40

1609 F06

–80

–100

100

–120

–130

–20

fSAMPLE = 200kHz

fIN = 1kHz

SINAD = 87.2dB

THD = –100.1dB

Total Harmonic Distortion

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. THD is

expressed as:

THD

VVV

log

...

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