Datasheet

LT1994
14
1994fb
A plot of this equation and a plot of the noise generated
by the feedback components are shown in Figure 6.
The LT1994’s input referred voltage noise contributes the
equivalent noise of a 560Ω resistor. When the feedback
network is comprised of resistors whose values are less
than this, the LT1994’s output noise is voltage noise
dominant (See Figure 6):
ee
R
R
no ni
F
I
≈+
⎛
⎝
⎜
⎞
⎠
⎟
•1
Feedback networks consisting of resistors with values
greater than about 10k will result in output noise which
is amplifi er current noise dominant.
eIR
no n F
≈ 2• •
Lower resistor values always result in lower noise at the
penalty of increased distortion due to increased loading of
the feedback network on the output. Higher resistor values
will result in higher output noise, but improved distortion
due to less loading on the output.
Figure 6 shows the noise voltage that will appear differ-
entially between the outputs. The common mode output
noise voltage does not add to this differential noise. For
optimum noise and distortion performance, use a dif-
ferential output confi guration.
Power Dissipation Considerations
The LT1994 is housed in either an 8-lead MSOP package
(θ
JA
= 140°C/W or an 8-lead DD package (θ
JA
= 43°C/W).
The LT1994 combines high speed and large output current
with a small die and small package so there is a need to
be sure the die temperature does not exceed 150°C. In the
8-lead MSOP, LT1994 has its V
–
lead fused to the frame
so it is possible to lower the package thermal impedance
by connecting the V
–
pin to a large ground plane or metal
trace. Metal trace and plated through holes can be used to
spread the heat generated by the device to the backside
of the PC board. For example, an 8-lead MSOP on a 3/32"
FR-4 board with 540mm
2
of 2oz. copper on both sides
of the PC board tied to the V
–
pin can drop the θ
JA
from
140°C/W to 110°C/W (see Table 1).
The underside of the DD package has exposed metal
(4mm
2
) from the lead frame where the die is attached.
This provides for the direct transfer of heat from the die
junction to the printed circuit board to help control the
maximum operating junction temperature. The dual-in-line
pin arrangement allows for extended metal beyond the
ends of the package on the topside (component side) of a
circuit board. Table 1 summarizes for the MSOP package,
the thermal resistance from the die junction-to-ambient
that can be obtained using various amounts of topside,
and backside metal (2oz. copper). On multilayer boards,
further reductions can be obtained using additional metal
on inner PCB layers connected through vias beneath the
package.
Figure 6. LT1994 Output Spot Noise vs Spot Noise Contributed by
Feedback Network Alone
R
F
= R
I
(k)
0.1
1
OUTPUT NOISE (nV/√Hz)
10
100
110
1994 F06
TOTAL
(AMPLIFIER + FEEDBACK NETWORK)
OUTPUT NOISE
FEEDBACK NETWORK
NOISE ALONE
1994 F05
R
I2
R
F2
V
S
/2
–V
S
/2
+
–
–
+
3
7
6
8
1
2
5
4
LT1994V
OCM
e
nRI2
2
R
I1
R
F1
e
ncm
2
e
no
2
i
n–
2
e
nRI1
2
e
ni
2
e
nRF2
2
e
nRF1
2
i
n+
2
Figure 5. Noise Analysis
APPLICATIONS INFORMATION