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LTC1598LCG

4- and 8-Channel, 3V Micropower Sampling 12-Bit Serial I/O A/D Converters

Linear Technology 

LTC1594L/LTC1598L

APPLICATIONS INFORMATION

Conversion Speed with Reduced VREF

With reduced reference voltages, the LSB step size is

reduced and the LTC1594L/LTC1598L internal compara-

tor overdrive is reduced. Therefore, it may be necessary

to reduce the maximum CLK frequency when low values

of VREF are used.

DYNAMIC PERFORMANCE

The LTC1594L/LTC1598L have exceptional sampling

capability. 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 LTC1594L/LTC1598L

plot.

0

TA = 25°C

VCC = 2.7V

– 20 VREF = 2.5V

fIN = 3.05kHz

– 40

fCLK = 120kHz

fSMPL = 7.5kHz

– 60

– 80

– 100

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 10.5kHz with a 5V supply, the LTC1594L/

LTC1598L maintain above 10.7 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.

12

74

11

68

10

62

9

56

8

50

7

6

5

4

3 TA = 25°C

2 VCC = 2.7V

1

fCLK = 200kHz

fSMPL = 10.5kHz

0

1

10

100

INPUT FREQUENCY (kHz)

1594L/98L G09

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

– 120

0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

FREQUENCY (kHz)

1594L/98L G13

Figure 10. LTC1594L/LTC1598L Nonaveraged,

4096 Point FFT Plot

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 10.5kHz sampling rate.

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 V22 + V32 + V42 + ... + VN2

V1

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

18

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