Datasheet

zero"" Gain Noise #
1
CEQpC
C)RR(R2 ++S
4.0
2.0
0.0
-2.0
-4.0
-6.0
f (MHz)
Normalized Gain (dB)
R
A
R
f
R
g
R
B
C
f
¶
C
f '
= 1.5 pF
R
A
= 33:
R
B
= 91:
1 10 100 1,000 10,000
LMH6629
SNOSB18G –APRIL 2010–REVISED MARCH 2013 www.ti.com
This circuit operates by increasing the Noise Gain (NG) beyond the minimum stable gain of the LMH6629 while
maintaining a positive loop gain phase angle at 0dB. There are two constraints shown in Figure 53; “Constraint
1” ensures that NG has increased to at least 10 V/V when the loop gain approaches 0dB, and “Constraint 2”
places an upper limit on the feedback phase lead network frequency to make sure it is fully effective in the
frequency range when loop gain approaches 0dB. These two constraints allow one to estimate the “starting
value” for R
c
and C
c
which may need to be fine tuned for proper response.
Here is an example worked out for more clarification:
Assume that the objective is to use the SOT-23-5 version of the LMH6629 for a closed loop gain of +3.7 V/V
using the technique shown in Figure 53.
Selecting R
f
= 249Ω → R
g
= 91Ω → R
EQ
= 66.6Ω.
For 50Ω source termination (R
s
= 50Ω), select R
T
= 50Ω → R
p
= 25Ω.
Using “Constraint 1” (= 10V/V) allows one to compute Rc ≊ 56Ω. Using “Constraint 2” (= 90 MHz) defines the
appropriate value of C
c
≊ 33 pF.
The frequency response plot shown in Figure 54 is the measured response with R
c
and C
c
values computed
above and shows a -3dB response of about 1GHz.
Figure 54. SOT-23-5 Package Low Closed Loop Gain Operation with External Compensation
For the Figure 54 measured results, a compensation capacitor (C
f'
) was used across R
f
to compensate for the
summing node net capacitance due to the board and the SOT-23–5 LMH6629. The R
A
and R
B
combination
reduces the effective capacitance of Cf‘ by the ratio of 1+R
B
/ R
A
, with the constraint that R
B
<< R
f
, thereby
allowing a practical capacitance value (> 1pF) to be used. The WSON-8 package does not need this
compensation across R
f
due to its lower parasitics.
With the COMP pin HI (WSON-8 package only) or with the SOT-23–5 package, this circuit achieves high slew
rate and takes advantage of the LMH6629’s superior low-noise characteristics without sacrificing stability, while
enabling lower gain applications. It should be noted that the R
c
, C
c
combination does lower the input impedance
and increases noise gain at higher frequencies. With these values, the input impedance reduces by 3dB at 490
MHz. The Noise Gain transfer function “zero” is given by the equation below and it has a 3dB increase at 32.8
MHz with these values:
Equation 1: External Compensation Noise Gain Increase (1)
CANCELLATION OF OFFSET ERRORS DUE TO INPUT BIAS CURRENTS
The LMH6629 offers exceptional offset voltage accuracy. In order to preserve the low offset voltage errors, care
must be taken to avoid voltage errors due to input bias currents. This is important in both inverting and non
inverting applications.
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