LTC1404 LINER [Linear Technology], LTC1404 Datasheet - Page 12

• Manufacturer Part Number: LTC1404
• Description: Complete SO-8, 12-Bit, 600ksps ADC with Shutdown
• Manufacturer: LINER [Linear Technology]

LTC1404
If two pure sine waves of frequencies fa and fb are applied
to the ADC input, nonlinearities in the ADC transfer func-
tion can create distortion products at sum and difference
frequencies of mfa nfb, where m and n = 0, 1, 2, 3, etc.
For example, the 2nd order IMD terms include (fa + fb) and
(fa – fb) while the 3rd order IMD terms includes (2fa + fb),
(2fa – fb), (fa + 2fb) and (fa – 2fb). If the two input sine
waves are equal in magnitude, the value (in decibels) of the
2nd order IMD products can be expressed by the following
formula.
Figure 5 shows the IMD performance at a 100kHz input.
Peak Harmonic or Spurious Noise
The peak harmonic or spurious noise is the largest spec-
tral component excluding the input signal and DC. This
value is expressed in decibels relative to the RMS value of
a full-scale input signal.
Full Power and Full Linear Bandwidth
The full power bandwidth is the input frequency at which
the amplitude of the reconstructed fundamental is re-
duced by 3dB for a full-scale input signal.
The full linear bandwidth is the input frequency at which
the S/(N + D) has dropped to 68dB (11 effective bits). The
12
APPLICATIONS
–100
–110
–120
–10
–20
–30
–40
–50
–60
–70
–90
–80
IMD fa fb
Figure 5. Intermodulation Distortion Plot in Bipolar Mode
0
0
20
40
2fa – fb
60
80 100
20log
U
fa
fb
120
INFORMATION
FREQUENCY (kHz)
Amplitude at (
2fb – fa
U
140
Amplitude at fa
160
180
2fa
W
fa + fb
200
fa fb
220
f
fa = 99.16992188kHz
fb = 102.6855469kHz
2fb
SAMPLE
240 260 280 300
U
)
= 600kHz
2fb + fa
2fa + fb
3fa
3fb
1404 F05
LTC1404 has been designed to optimize input bandwidth,
allowing the ADC to undersample input signals with fre-
quencies above the converter’s Nyquist Frequency. The
noise floor stays very low at high frequencies; S/(N + D)
becomes dominated by distortion at frequencies far be-
yond Nyquist.
Driving the Analog Input
The analog input of the LTC1404 is easy to drive. It draws
only one small current spike while charging the sample-
and-hold capacitor at the end of a conversion. During
conversion, the analog input draws only a small leakage
current. The only requirement is that the amplifier driving
the analog input must settle after the small current spike
before the next conversion starts. Any op amp that settles
in 160ns to small load current transient will allow maxi-
mum speed operation. If a slower op amp is used, more
settling time can be provided by increasing the time
between conversions. Suitable devices capable of driving
the ADC’s A
op amps.
The LTC1404 comes with a built-in unipolar/bipolar detec-
tion circuit. If the V
internal circuitry will automatically switch to bipolar mode.
The following list is a summary of the op amps that are
suitable for driving the LTC1404, more detailed informa-
tion is available in the Linear Technology databooks and
the LinearView
LT 1215/LT1216: Dual and quad 23MHz, 50V/ s single
supply op amps. Single 5V to 15V supplies, 6.6mA
specifications, 90ns settling to 0.5LSB.
LT1223: 100MHz video current feedback amplifier. 5V
to 15V supplies, 6mA supply current. Low distortion up
to and above 600kHz. Low noise. Good for AC applica-
tions.
LT1227: 140MHz video current feedback amplifier. 5V
to 15V supplies, 10mA supply current. Lowest distor-
tion at frequencies above 600kHz. Low noise. Best for AC
applications.
IN
input include the LT
TM
CD-ROM.
SS
potential is forced below GND, the
®
1360 and the LT1363