Datasheet

LMP8350
SNOSB80B –FEBRUARY 2011–REVISED MARCH 2013 www.ti.com
APPLICATION SECTION
The LMP8350 is a fully differential voltage feedback amplifier designed to drive precision differential ADC
converters. The LMP8350, though fully integrated for ultimate balance and distortion performance, functionally
provides three channels. Two of these channels are the V
+
and V
−
signal path channels, which function similarly
to inverting mode operational amplifiers and are the primary signal paths.
The third “channel” is the common mode (V
OCM
) feedback circuit. This is the circuit that sets the output common
mode as well as driving the V
+
and V
−
outputs to be equal magnitude and opposite phase, even when only one
of the two input channels is driven. The common mode feedback circuit allows for single ended to differential
operation. The output common mode voltage is set by applying the appropriate voltage to the V
OCM
pin.
ENABLE PIN AND POWER MODE SELECTION
The LMP8350 is equipped with a four level enable (EN) pin to select one of three power modes or shutdown.
These modes are selected by applying the appropriate voltage to the EN pin.
Each power level has a corresponding performance level. The high power mode will have the best overall BW
and distortion performance, but at the cost of higher supply current and some DC accuracy. The Low power
mode has the lowest supply current, but with a noticeable loss of AC performance and output drive capabilities.
The mid-power mode provides the best balance of AC and precision DC specifications. In disable mode, the
amplifier is shutdown and the output stage goes into a high impedance state. Table 1 summarizes these
performance trade-offs.
Table 1. Performance vs. Power Mode Summary
Mode V
S
–3dB BW HD2 Noise SR Typ Vos
(MHz) (dBc) (nV/Hz) (V/µS) (mV)
10 118 –124.7 4.6 507 0.6
High 6.6 116 –124.7 4.5 488 0.3
5 114 –125.5 4.5 476 0.2
10 87 –122.8 4.8 393 0.08
Med 6.6 85 –122.8 4.8 376 0.1
5 84 –122.6 4.8 366 0.1
10 31 –117.2 8.0 178 0.1
Low 6.6 29 –117.2 8.0 166 0.1
5 28 –117.0 8.0 160 0.1
To set the mode, internally the voltage at the EN pin is compared against the total supply voltage (V
S
) and sets
the current consumption as shown in the table below. The EN pin voltage is referenced to the V- pin.
Table 2. Enable Pin Mode Selection
V
EN
Power V
EN
@ 10V V
EN
@ 6.6V V
EN
@ 5V I
S
(V
S
= V+ - V-) Mode mA
7/8 * V
S
High 8.75 5.775 4.375 13 to 15
5/8 * V
S
Med 6.25 4.125 3.125 7 to 9
3/8 * V
S
Low 3.75 2.475 1.875 2 to 3
1/8 * V
S
Dis 1.25 0.825 0.625 <1
The enable pin should not be allowed to float. If the enable pin is not used it can be tied to V+ to select the high
power mode or set with two resistors.
Each power setting has a +/-400mV tolerance at each level, though it is recommended to keep the set voltage
within the center of the range as performance may vary near the transition zones.
During shutdown, both outputs are in a high impedance state, so the feedback and gain set resistors will then set
the input and output impedance of the circuit. For this reason input to output isolation will be poor in the disabled
state.
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