ltc1867lign-trpbf Linear Technology Corporation, ltc1867lign-trpbf Datasheet - Page 11

• Manufacturer Part Number: ltc1867lign-trpbf
• Description: Ltc1863 - 12-bit, 8-channel 200ksps Adcs
• Manufacturer: Linear Technology Corporation

APPLICATIO S I FOR ATIO
DC Performance
One way of measuring the transition noise associated with
a high resolution ADC is to use a technique where a DC
signal is applied to the input of the ADC and the resulting
output codes are collected over a large number of conver-
sions. For example, in Figure 2 the distribution of output
codes is shown for a DC input that had been digitized 4096
times. The distribution is Gaussian and the RMS code
transition noise is about 0.74LSB.
Figure 1a. Optional RC Input Filtering for Single-Ended Input
Figure 1b. Optional RC Input Filtering for Differential Inputs
Figure 2. LTC1867 Histogram for 4096 Conversions
DIFFERENTIAL
ANALOG
INPUT
ANALOG
INPUTS
2500
2000
1500
1000
500
0
–4
1
50
–3
26
U
50
50
276
–2
935
–1
U
2000pF
10 F
2152
CODE
1000pF
1000pF
1000pF
10 F
0
579
1
CH0
GND
REFCOMP
CH0
CH1
W
REFCOMP
122
2
LTC1863/
LTC1867
LTC1863/
LTC1867
5
3
18637 GO3
0
4
1867 F01a
1867 F01b
U
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 algo-
rithm, the ADC’s spectral content can be examined for
frequencies outside the fundamental.
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 3 shows a typical SINAD of 87.9dB with
a 200kHz sampling rate and a 1kHz input. When an
external 5V is applied to REFCOMP (tie V
signal-to-noise ratio of 90dB can be achieved.
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
Figure 3. LTC1867 Nonaveraged 4096 Point FFT Plot
20
–100
–120
–140
–20
–40
–60
–80
log
0
0
V
2
LTC1863/LTC1867
2
25
FREQUENCY (kHz)
V
3
2
V
50
SNR = 88.8dB
SINAD = 87.9dB
THD = 95dB
f
INTERNAL REFERENCE
SAMPLE
1
V
4
2
...
= 200ksps
75
V
N
18637 G04
2
REF
100
to GND), a
11
18637f