LMV551_15 Datasheet, PDF (13/26 Pages) Texas Instruments – LMV551/LMV552/LMV554 3 MHz, Micropower RRO Amplifiers

LMV551, LMV552
www.ti.com
SNOSAQ5G – FEBRUARY 2007 – REVISED FEBRUARY 2013
R_S = R_OUTR_IN
R_F
C_F =
¨¨©§¹
+
1
A_CL
¨¨©§
R_F
+ 2R_IN
R_F2
C_LR_OUT
(1)
C_L (pF)
50
100
150
R_S (Ω)
340
340
340
C_F (pF)
8
15
22
Table 1. Phase Margins
Phase Margin (°)
47
42
40
Although this methodology provides circuit stability for any load capacitance, it does so at the price of bandwidth.
The closed loop bandwidth of the circuit is now limited by R_F and C_F.
Compensation by External Resistor
In some applications it is essential to drive a capacitive load without sacrificing bandwidth. In such a case, in the
loop compensation is not viable. A simpler scheme for compensation is shown in Figure 32. A resistor, R_ISO, is
placed in series between the load capacitance and the output. This introduces a zero in the circuit transfer
function, which counteracts the effect of the pole formed by the load capacitance and ensures stability. The value
of R_ISO to be used should be decided depending on the size of C_L and the level of performance desired. Values
ranging from 5Ω to 50Ω are usually sufficient to ensure stability. A larger value of R_ISO will result in a system with
less ringing and overshoot, but will also limit the output swing and the short circuit current of the circuit.
Figure 32. Compensation by Isolation Resistor
TYPICAL APPLICATION
ACTIVE FILTERS
With a wide unity gain bandwidth of 3 MHz, low input referred noise density and a low power supply current, the
LMV551/LMV552/LMV554 are well suited for low-power filtering applications. Active filter topologies, such as the
Sallen-Key low pass filter shown in Figure 33, are very versatile, and can be used to design a wide variety of
filters (Chebyshev, Butterworth or Bessel). The Sallen-Key topology, in particular, can be used to attain a wide
range of Q, by using positive feedback to reject the undesired frequency range.
In the circuit shown in Figure 33, the two capacitors appear as open circuits at lower frequencies and the signal
is simply buffered to the output. At high frequencies the capacitors appear as short circuits and the signal is
shunted to ground by one of the capacitors before it can be amplified. Near the cut-off frequency, where the
impedance of the capacitances is on the same order as R_G and R_F, positive feedback through the other capacitor
allows the circuit to attain the desired Q.
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