LT1994 Linear Technology, LT1994 Datasheet - Page 16

• Manufacturer Part Number: LT1994
• Description: Low Noise - Low Distortion Fully Differential Input/Output Amplifier/Driver
• Manufacturer: Linear Technology

APPLICATIO S I FOR ATIO
SI PLIFIED
LT1994
Any stray parasitic capacitance to ground at the summing
junctions, IN
mum even if it means stripping back the ground plane
away from any trace attached to this node. This becomes
especially true when the feedback resistor network uses
resistor values >500Ω in circuits with R
peaking in the frequency response can be mitigated by
adding small amounts of feedback capacitance around RF
(2pF to 5pF). Always keep in mind the differential nature of
the LT1994, and that it is critical that the output impedances
seen by both outputs (stray or intended) should be as bal-
anced and symmetric as possible. This will help preserve
the natural balance of the LT1994, which minimizes the
generation of even order harmonics, and preserves the
rejection of common mode signals and noise.
16
W
OUT
IN
IN
V
+
+
–
–
+
and IN
V
V
V
V
V
V
U
SCHE ATIC
+
–
+
–
+
–
–
should be kept to an absolute mini-
U
Q10
W
Q9
V
V
+
–
Q1
W
ADJUST
BIAS
D1
D3
C
F
M1
V
+
= R
D2
D4
I2
I
. Excessive
U
Q2
Gm
+
–
2B
Q3
BIAS
I1
Q4
It is highly recommended that the V
tied to a low impedance ground plane (in split supply
applications) or bypassed to ground with a high quality
0.1μF ceramic capacitor in single supply applications.
This will help prevent thermal noise from the internal
80kΩ-80kΩ voltage divider (25nV/√Hz) and other exter-
nal sources of noise from being converted to differential
noise due to mismatches in the feedback networks. It is
also recommended that the resistive feedback networks
be comprised of 1% resistors (or better) to enhance the
output common mode rejection. This will also prevent
V
mode amplifi er path (which cannot be fi ltered) from being
converted to differential noise, degrading the differential
noise performance.
OCM
I1
Gm
–
+
2A
ADJUST
input referred common mode noise of the common
CM
R1
R2
4k
4k
OUT
OUT
+
–
SHUTDOWN
+
CIRCUIT
V
Q7
C
ADJUST
M2
BIAS
I4A
Q5 Q6
V
+
I3
Q8
V
V
V
–
+
Q11
Q12
+
I4B
V
V
+
–
120k
55k
80k
80k
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OCM
V
V
V
V
V
V
+
–
+
–
+
–
1994 SS01
pin be either hard
SHDN
OUT
V
V
OCM
–
+
1994fa