Datasheet

LOW IMPEDANCE
VOLTAGE REFERENCE
V
I
R
G1
R
G2
V
OCM
R
F1
R
F2
EN
A/D
R
ISO1
C
S
8-18 pF
+
-
a
V
REF
R
ISO2
LMP8350
SNOSB80B –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
Figure 37. Driving an ADC
The amplifier and ADC should be located as close together as possible. Both devices require that the filter
components be in close proximity to them. The amplifier needs to have minimal parasitic loading on the output
traces, and the ADC is sensitive to high frequency noise that may couple in on its input lines. Some high
performance ADCs have an input stage that has a bandwidth of several times its sample rate. The sampling
process results in all input signals presented to the input stage mixing down into the Nyquist range (DC to Fs/2).
See AN-236 (SNAA079) for more details on the subsampling process and the requirements this imposes on the
filtering necessary in your system.
CAPACITIVE DRIVE
As noted in the Driving ADC section, capacitive loads should be isolated from the amplifier output with small
valued resistors. This is particularly the case when the load has a resistive component that is 500Ω or higher. A
typical ADC has capacitive components of around 8 to 18pF, and the resistive component could be 1000Ω or
higher. If driving a transmission line, such as a twisted pair, using matching resistors will be sufficient to isolate
any subsequent capacitance.
POWER DISSIPATION
The LMP8350 is optimized for maximum performance in the small form factor of the standard SOIC package,
and is essentially a dual channel amplifier. To ensure maximum output drive and highest performance, thermal
shutdown is not provided. Therefore, it is of utmost importance to make sure that the T
JMAX
of 150°C is never
exceeded due to the overall power dissipation.
Follow these steps to determine the Maximum power dissipation for the LMP8350:
1. Calculate the quiescent (no-load) power: P
AMP
= I
CC
* (V
S
), where V
S
= V
+
- V
−
. (Be sure to include any
current through the feedback network if V
OCM
is not mid rail.)
2. Calculate the RMS power dissipated in each of the output stages: P
D
(rms) = rms ((V
S
- V
+
OUT
) * I
+
OUT
) + rms
((V
S
− V
−
OUT
) * I
−
OUT
) , where V
OUT
and I
OUT
are the voltage and the current measured at the output pins of
the differential amplifier as if they were single ended amplifiers and V
S
is the total supply voltage.
3. Calculate the total RMS power: P
T
= P
AMP
+ P
D
.
The maximum power that the LMP8350 package can dissipate at a given temperature can be derived with the
following equation:
P
MAX
= (150° – T
AMB
)/ θ
JA
, where T
AMB
= Ambient temperature (°C) and θ
JA
= Thermal resistance, from junction
to ambient, for a given package (°C/W). For the SOIC package θ
JA
is 150°C/W.
NOTE: If V
OCM
is not 0V then there will be quiescent current flowing in the feedback network. This current should
be included in the thermal calculations and added into the quiescent power dissipation of the amplifier.
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