LMH6401_16 Datasheet, PDF (29/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

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LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
10.1.3.1 Step Response Optimization using Thermal Feedback Control
The LMH6401 has an adjustable frequency compensation scheme that is designed to dramatically improve the
step response for long-term settling. The structure of the LMH6401 gives the best distortion performance for
signals ranging from dc to 2 GHz over a wide range of gain settings. Thermal heating causes a small change in
gain at low frequencies close to 500 kHz. This change in gain is shown in Figure 61 in the ac response for the
trace labeled Thermal Feedback OFF. The amount of gain change is approximately 0.18 dB at maximum gain.
This gain change resulting from thermal heating leads to approximately 1.7% overshoot that settles over a
relatively long time period. A patent pending technique is added that allows for the reduction of this overshoot to
approximately 0.35%, thus eliminating the long-term settling and still retaining the wide dynamic performance
range. The circuit also corrects for small systematic changes that occur at different gain settings and tracks
temperature changes as well. This low-frequency gain correction is accomplished by the addition of a circuit that
alters the gain at low frequencies to nearly eliminate the variation from low to high frequencies.
The step response optimization circuit is disabled on power-up and can be enabled by clearing bit 5 in the
Thermal Feedback Gain Control register (register 4h). The power-on default setting for thermal gain and
frequency are adjusted for the evaluation board for typical silicon performance. These registers are made
available for customization in the final system because board layout characteristics or other components in the
system can change the required correction needed.
Figure 63 demonstrates the initial step response and the corrected response that corresponds to the default
register values for a typical device on the evaluation board displaying long-term settling correction.
26.2
26.16
26.12
26.08
Max Gain
26.04
26
25.96
25.92
25.88
25.84
Min Gain
Thermal Feedback OFF
25.8
1
10
100
1000
10000 100000
Frequency (kHz)
D047
Figure 61. Thermal Gain Control Sweep (Frequency = 9)
1.02
1.015
26.16
26.12
26.08
Max Frequency
26.04
26
25.96
25.92
Min Frequency
25.88
1
10
100
1000
10000 100000
Frequency (kHz)
D048
Figure 62. Thermal Frequency Control Sweep (Gain = 10)
Uncorrected
Corrected
1.01
1.005
1
0.995
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
Time (usec)
D049
Figure 63. Long-Term Settling Response using Thermal Feedback
(Gain = 10, Frequency = 9)
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