lt1394cs8-trpbf Linear Technology Corporation, lt1394cs8-trpbf Datasheet - Page 9

• Manufacturer Part Number: lt1394cs8-trpbf
• Description: 7ns, Low Power, Single Supply, Ground-sensing Comparator
• Manufacturer: Linear Technology Corporation

5V
APPLICATIONS
Figure 3. Crystal Oscillators for Outputs to 30MHz. Circuit (b)’s
Damper Network Supresses Overtone Crystal’s Harmonic Modes
1k
1k
Figure 4. Switchable Output Crystal Oscillator. Biasing A or B
High Places Associated Crystal in Feedback Path. Additional
Crystal Branches Are Permissible
(a)
(b)
75pF
+
–
LT1394
XTAL B
XTAL A
XTAL X
22
820pF
GROUND XTAL CASES
5V
U
5V
2k
2k
2k
2k
= 1N4148
2k
D1
INFORMATION
U
0.068 F
200pF
CRYSTAL (AT-CUT)
CRYSTAL (AT-CUT)
+
–
+
–
10MHz TO 25MHz
1MHz TO 10MHz
LT1394
LT1394
D2
2k
2k
W
D
X
1k
1k
R
X
B
A
OUTPUT
LOGIC INPUTS
AS MANY STAGES
AS DESIRED
OUTPUT
OUTPUT
U
1394 F03
1394 F04
Temperature-Compensated Crystal Oscillator (TXCO)
Figure 5 is a temperature-compensated crystal oscillator
(TXCO). This circuit reduces oscillator temperature drift
by inserting a temperature-dependent compensatory cor-
rection into the crystal’s frequency trimming network.
This open-loop correction technique relies on cancellation
of the temperature characteristics of the oscillator, which
are quite repeatable.
The LT1394 and associated components form the crystal
oscillator, operating similarly to Figure 3’s examples. The
LM134, a temperature-dependent current source, biases
A1. A1 takes gain referred to the LM134’s output and the
negative offset supplied via the 470k -LT1004 reference
path. Note that the LT1004’s negative voltage bias is
bootstrapped from the oscillator’s output, maintaining
single supply operation. This arrangement delivers tem-
perature-dependent bias to the varactor diode, causing a
scaled variation in the crystal’s resonance versus ambient
temperature. The varactor’s bias-dependent capacitance
shift pulls crystal frequency to complement the circuit’s
temperature drift. The simple first order fit provided by the
compensation is very effective. Figure 6 shows results.
The –70ppm frequency shift over 0 C to 70 C is corrected
within a few ppm. The “FREQ SET” trim also biases the
varactor, allowing accurate output frequency setting. It is
worth noting that better compensation is possible by
including higher order terms in the temperature-to-volt-
age conversion.
18ns, 500 V Sensitivity Comparator
The ultimate limitation on comparator sensitivity is avail-
able gain. Unfortunately, increasing gain invariably
involves giving up speed. The gain vs. speed trade-off in a
fast comparator is usually a practical compromise
designed to satisfy most applications. Some situations,
however, require more sensitivity (e.g., higher gain) with
minimal impact on speed. Figure 7’s circuit adds a differ-
ential preamplifier ahead of the LT1394, increasing gain.
This permits 500 V comparisons in 18ns. A parallel path
DC stabilization approach eliminates preamplifier drift as
an error source. A1 is the differential preamplifier, operat-
ing at a gain of 100. Its output is AC-coupled to the LT1394.
LT1394
9