LMH2180 Datasheet, PDF (14/24 Pages) National Semiconductor (TI) – 75 MHz Dual Clock Buffer

LMH2180
SNAS419D – JANUARY 2008 – REVISED MARCH 2013
VCTCXO
C1
LMH2180
OUT 1
IN 1
Isolation
IN 2
OUT 2
Base
band IC
Bluetooth
WLAN
Figure 37. Isolation Block Diagram
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DRIVING CAPACITIVE LOADS
Each buffer can drive a capacitive load. Be aware that every capacitor directly connected to the output becomes
part of the loop of the buffer. In most applications the load consists of the capacitance of copper tracks and the
input capacitance of the application blocks. Capacitance reduces the gain/phase margin and decreases the
stability. This leads to peaking in the frequency response and in extreme situations oscillations can occur. To
drive a large capacitive load it is recommended to include a series resistor between the buffer and the load
capacitor. The best value for this isolation resistance can be found by experimentation.
The LMH2180 datasheet reflects measurements with capacitive loads of 10 pF at the output of the buffers. Most
common applications will probably use a lower capacitive load, which will result in lower peaking and significantly
greater bandwidth, see Figure 38.
9
33 pF
6
22 pF
10 pF
3
0
-3
6.8 pF
-6
No C Load
-9
-12
-15 _{VIN = 0.1VPP}
-18
1M
10M
100M
FREQUENCY (Hz)
Figure 38. Bandwidth and Peaking
PHASE NOISE
A clock buffer adds noise to the clock signal. This noise causes uncertainty in the phase of the clock signal. This
uncertainty is described by jitter (time domain) or phase noise (frequency domain). Communication systems,
such as Wireless LAN, require a low jitter/phase noise clock signal to obtain a low Bit Error Rate. Figure 39
shows the frequency domain representation of a clock signal with frequency f_C. Without Phase Noise the entire
signal power would only be located at the frequency f_C. Phase Noise spreads some of the power to adjacent
frequencies. Phase Noise is usually specified in dBc/Hz at a given frequency offset Δf from the carrier, where
dBc is the power level in dB relative to the carrier. The noise power is measured within a 1 Hz bandwidth.
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