LMP7709MA/NOPB National Semiconductor, LMP7709MA/NOPB Datasheet - Page 18

IC AMP RRIO PREC QUAD 14SOIC

LMP7709MA/NOPB

Manufacturer Part Number
LMP7709MA/NOPB
Description
IC AMP RRIO PREC QUAD 14SOIC
Manufacturer
National Semiconductor
Series
LMP®, PowerWise®r
Datasheet

Specifications of LMP7709MA/NOPB

Amplifier Type
General Purpose
Number Of Circuits
4
Output Type
Rail-to-Rail
Slew Rate
5.9 V/µs
Gain Bandwidth Product
15MHz
Current - Input Bias
0.2pA
Voltage - Input Offset
37µV
Current - Supply
3.2mA
Current - Output / Channel
86mA
Voltage - Supply, Single/dual (±)
2.7 V ~ 12 V, ±1.35 V ~ 6 V
Operating Temperature
-40°C ~ 125°C
Mounting Type
Surface Mount
Package / Case
14-SOIC (3.9mm Width), 14-SOL
Lead Free Status / RoHS Status
Lead free / RoHS Compliant
-3db Bandwidth
-
Other names
*LMP7709MA/NOPB
LMP7709MA
www.national.com
The feedback function for a three-terminal op amp as shown
in Figure 8 is the feedback voltage V
input terminals relative to the op amp output voltage, V
That is
GRAPHICAL EXPLANATION OF STABILITY ANALYSIS
Stability issues can be observed by verifying the closed loop
gain function GF. In the frequencies of interest, the open loop
gain (G) of the amplifier is a number larger than 1 and there-
fore positive in dB. The feedback factor (F) of the feedback
circuit is an attenuation and therefore negative in dB. For cal-
culating the closed loop gain GF in dB we can add the values
of G and F (both in dB).
One practical approach to stabilizing the system, is to assign
a value to the feedback factor F such that the remaining loop
gain GF equals one (unity gain) at the frequency of G
realizes a phase margin of 45° or greater. This results in the
following requirement for stability: 1/F > G
feedback factor 1/F is constant over frequency and should
intercept the open loop gain at a value in dB that is greater
than or equal to G
The inverse feedback factor for both configurations shown in
Figure 8, is given by:
The closed loop gain for the non-inverting configuration (a) is:
The closed loop gain for the inverting configuration (b) is:
For stable operation the phase margin must be equal to or
greater than 45° . The corresponding closed loop gain G
for a non-inverting configuration, is
For an inverting configuration:
If R
lower than the minimum gain required for stability, then 1/F
intersects the open loop gain curve for a value that is lower
than G
FIGURE 8. Op Amp with Resistive Feedback. (a) Non-
1
and R
MIN
. For example, assume the G
F
and are chosen so that the closed loop gain is
MIN
inverting (b) Inverting
.
A
– V
MIN
B
across the op amp
is equal to 10 V/V
MIN
. The inverse
MIN
. This
20203796
OUT
MIN
(3)
(4)
(5)
(6)
(7)
(8)
.
,
18
(20 dB). This is shown as the dashed line in Figure 9. The
resistor choice of R
equal 2 V/V (6 dB), shown in Figure 9 as the solid line. The
intercept of G and 1/F represents the frequency for which the
loop gain is identical to 1 (0 dB). Consequently, the total phase
shift at the frequency of this intercept determines the phase
margin and the overall system stability. In this system exam-
ple 1/F crosses the open loop gain at a frequency which is
larger than the frequency where G
system has less than 45° phase margin and is most likely in-
stable.
RESISTIVE COMPENSATION
A straightforward way to achieve a stable amplifier configu-
ration is to add a resistor R
non-inverting inputs as shown in Figure 10.
This additional resistor R
of the amplifier but it will have positive impact on the feedback
network.
The inverse feedback function of this circuit is:
Proper selection of the value of R
the 1/F function to G
for circuit stability. The compensation technique of reducing
the loop gain may be used to stabilize the circuit for the values
given in the previous example, that is G
R
amplifier inputs shifts the 1/F curve to the value G
as shown by the dashed line in Figure 11. This results in
overall stability for the circuit. This figure shows a combination
of the open and closed loop gain and the inverse feedback
function.
1
FIGURE 9. 1/F for R
= 2 kΩ. A resistor value of 250 Ω applied between the
FIGURE 10. Op Amp with Compensation Resistor
F
MIN
= R
between Inputs
F
or greater, thus fulfilling the condition
1
C
= R
= 2 kΩ makes the inverse feedback
will not affect the closed loop gain
C
1
between the inverting and the
and Open Loop Gain Plot
C
MIN
results in the shifting of
occurs, therefore this
MIN
20203797
= 20 dB and R
MIN
(20 dB)
202037a2
F
(9)
=

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