ltc6360 Linear Technology Corporation, ltc6360 Datasheet - Page 14

• Manufacturer Part Number: ltc6360
• Description: Very Low Noise Single-ended Sar Adc Driver With True Zero Output
• Manufacturer: Linear Technology Corporation

LTC6360
The charge pump is capable of sinking up to 4.5mA of
DC current with a typical DC output impedance of 30Ω. If
more current is demanded of the charge pump, the volt-
age at CPO will collapse towards 0V. A diode connected
from CPO to GND limits the CPO node from being pulled
above ground by more than one diode drop.
Transient currents are absorbed by the filter capacitors
from CPO/CPI to GND. Care should be taken in selecting
the filter capacitors such that there is minimum ripple
voltage and droop during peak transient current demand.
Using multiple small surface mount capacitors is ad-
vised, with each capacitor covering a portion of the total
frequency range.
Slew Rate and Full Power Bandwidth
Additional consideration needs to be paid to the current
demanded of the charge pump. When driving a capaci-
tive load, the LTC6360 will exhibit a clipped distortion
characteristic at a lower frequency than where slew rate
limited distortion would occur. In contrast to a traditional
amplifier, where the full power bandwidth is determined
from the amplifier’s slew rate, when driving capacitive
loads, the full power bandwidth of the LTC6360 will be
limited by the charge pump sinking capability.
The average current sunk by the charge pump when driving
a capacitive load can be approximated as:
where V
driven signal respectively.
The maximum frequency that the charge pump can support
while maintaining the CPO pin below –0.4V is:
where I
signals beyond this frequency will cause the charge pump
to collapse towards 0V, limiting the output amplitude and
causing distortion.
14
applicaTions inForMaTion
I
f
CP(AVG)
FPBW
CP(MAX)
P
= (I
and f are the amplitude and frequency of the
= 2V
CP(MAX)
is given in the specification table. Full-scale
P
• C
FILT
– 1mA)/(2V
• f + 1mA
P
• C
FILT
)
(2)
(1)
Output Compensation
The LTC6360 is internally compensated to be gain of 5
stable. Lower gains require an external RC network at
the output to provide compensation. The amplifier has
been decompensated to provide the highest possible
gain-bandwidth with a typical RC load of 10Ω in series
with 330pF. The extra gain-bandwidth obtained serves to
reduce distortion over a wider bandwidth. Since an external
RC filter network is desired in most ADC applications, the
decompensation is transparent in these cases and actually
serves to improve distortion performance.
The RC network at the output contributes a pole-zero pair
that reduces the loop gain above the pole frequency. The
simplified circuit model at high frequencies is shown in
Figure 7. At high frequencies, the open-loop output imped-
ance of the amplifier can be represented by an equivalent
resistor, ro, of 45Ω.
The pole frequency is:
The zero frequency is:
which is also the –3dB bandwidth of the filter formed by
R
FILT
f
f
f
P
Z
Z
Figure 7. Pole-Zero Introduced by RC Network at Output
/f
= 1/(2π(R
= 1/(2πR
and C
P
= 1 + ro/R
FILT
TO FEEDBACK
FILT
AMPLIFIER
NETWORK
. The zero-pole ratio is given by:
FILT
V
+
–
O
C
FILT
FILT
+ ro)C
ro
)
FILT
f
fρ = 1/[2π(R
Z
OUT
)
= 1/[2πR
R
FILT
FILT
FILT
C
6360 F07
C
FILT
FILT
+ ro)C
]
FILT
]
6360f
(3)
(4)
(5)