LTC1594CS datasheet(18/24 Pages) LINER

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LTC1594/LTC1598

15948fb

Conversion Speed with Reduced VREF

With reduced reference voltages, the LSB step size is

reduced and the LTC1594/LTC1598 internal comparator

overdrive is reduced. Therefore, it may be necessary to

reduce the maximum CLK frequency when low values of

VREF are used.

DYNAMIC PERFORMANCE

The LTC1594/LTC1598 have exceptional sampling capa-

bility. Fast Fourier Transform (FFT) test techniques are

used to characterize 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 con-

tent can be examined for frequencies outside the funda-

mental. Figure 10 shows a typical LTC1594/LTC1598

plot.

APPLICATIONS INFORMATION

Effective Number of Bits

The Effective Number of Bits (ENOBs) is a measurement of

the resolution of an ADC and is directly related to S/(N + D)

by the equation:

ENOB = [S/(N + D) – 1.76]/6.02

where S/(N + D) is expressed in dB. At the maximum

sampling rate of 16.8kHz with a 5V supply, the LTC1594/

LTC1598 maintain above 11 ENOBs at 10kHz input

frequency. Above 10kHz the ENOBs gradually decline, as

shown in Figure 11, due to increasing second harmonic

distortion. The noise floor remains low.

FREQUENCY (kHz)

–60

–40

1594/98 G14

–80

–100

–120

–140

–20

TA = 25°C

VCC = VREF = 5V

fIN = 5kHz

fCLK = 320kHz

fSMPL = 12.5kHz

Figure 10. LTC1594/LTC1598 Nonaveraged, 4096 Point FFT Plot

INPUT FREQUENCY (kHz)

100

1000

1594/98 G10

TA = 25°C

VCC = 5V

fCLK = 320kHz

fSMPL = 16.8kHz

Figure 11. Effective Bits and S/(N + D) vs Input Frequency

Total Harmonic Distortion

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

sum of all harmonics of the input signal to the fundamen-

tal itself. The out-of-band harmonics alias into the fre-

quency band between DC and half of the sampling

frequency. THD is defined as:

THD

+++ +

20log

VVV

...

where V1 is the RMS amplitude of the fundamental

frequency and V2 through VN are the amplitudes of the

second through the Nth harmonics. The typical THD

Signal-to-Noise Ratio

The Signal-to-Noise plus Distortion Ratio (S/N + D) is the

ratio between the RMS amplitude of the fundamental

input frequency to the RMS amplitude of all other fre-

quency components at the ADC’s output. The output is

band limited to frequencies above DC and below one half

the sampling frequency. Figure 11 shows a typical spec-

tral content with a 16.8kHz sampling rate.

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