LMH6518_15 Datasheet, PDF (25/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

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LMH6518
SNOSB21C – MAY 2008 – REVISED JULY 2013
0.7 V_PP, 1.2V DC
LMH6518
0.43 V_PP, 1.2V DC
+5V
-5V
0.35 V_PP, 0V DC
R₃
R₁
50: _+OUT
V_x R₂ VOUT
R₂
50:
41.4:
-OUT
R₃
131.3:
To ADC
R₁
172.7:
+5V
-5V
Figure 61. Output CM Shift Scheme
With the scheme of Figure 61, Vx is kept at 1.2V, by proper selection of external resistor values, so that the
LMH6518 outputs are not CM-loaded. As was the case with input level shifting, this output level shifting also
consumes additional power (0.58W).
Output Swing, Clamping, and Operation Beyond Full Scale
One of the major concerns in interfacing to low voltage ADC’s (such as the Gsample/s ADC’s that the LMH6518
is intended to drive) is ensuring that the ADC input is not violated with excessive drive. For this reason, plus the
very important requirement of an oscilloscope to recover quickly and gracefully from an overdrive condition, the
LMH6518 is fitted with three overvoltage clamps; one at the Preamp output and one at Main and Auxiliary
outputs each. The Preamp clamp is responsible for preventing the Preamp from saturation (to minimize recovery
time) with large ladder attenuation when Preamp output swing is at its highest. On the other hand, the output
clamps, perform this function when the Ladder attenuation is lower and hence the output amplifier is closer to
saturation, and prolonged recovery, if not properly clamped. The combination of these clamps results in
Figure 51, Figure 52, Figure 53, and Figure 54 where it is possible to observe where output limiting starts due to
the clamp action. LMH6518 owes its fast recovery time (< 5 ns) from 50% overdrive to the said clamps.
Figure 51, Figure 52, Figure 53, and Figure 54, in Typical Performance Characteristics, can be used to determine
the LMH6518 linear swing beyond full scale. This information sets the overdrive limit for both oscilloscope
waveform capture and for signal triggering. The Preamp clamp is set tighter than the output clamp, evidenced by
lower output swing with 20 dB Ladder attenuation than with 0 dB. With high ladder attenuation (20 dB) defining
the limit, the graphs show that the “+Out” and “−Out” difference of 0.4V is well inside the clamp range, thereby
ensuring 0.8 V_PP of unhindered output swing. This corresponds to an overdrive capability of approximately ±7%
beyond full scale.
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