LMH6518_15 Datasheet, PDF (30/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
SNOSB21C – MAY 2008 – REVISED JULY 2013
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Table 7. Ladder Attenuation Data Field (continued)
Ladder Attenuation
Ladder Attenuation (dB)
1
1
1
1
Unallowed
NOTE
An “Unallowed” SPI-1 state may result in undefined operation where device behavior is
not ensured.
OSCILLOSCOPE TRIGGER APPLICATIONS
With the Auxiliary output of the LMH6518 offering a second output that follows the Main one (except for a slightly
reduced distortion performance), the oscilloscope trigger function can be implemented by tapping this output. The
“V_{CM_Aux}” input of the LMH6518 allows the Auxiliary common mode to be set. The trigger function can be
physically located at a distance from the main signal path, if need be, by taking advantage of the differential
Auxiliary output and rejecting any board related common mode interference pick-up at the receive end.
If Trigger circuitry is physically close to the LMH6518, the circuit diagram shown in Figure 68 allows operation
using only one of two Auxiliary outputs. The unused output does need to be terminated properly using R₁, R₁₁
combination. U3 (DAC101C085) generates a 0- 2.5V trigger level, with 2.4 mV resolution
2.5V
(= ₂₁₀ )
(15)
or 0.7% (= 2.4 mV x 100/0.35 V_PP) of FS, which is compared to the LMH6518 “+Out Aux” by using an ultra-fast
comparator, U2 (LMH7220). U2’s complimentary LVDS output is terminated in the required 100Ω load (R₁₀), for
best performance, where the LVDS Trigger output is available. The LMH7220’s offset voltage (±9.5 mV) and
offset voltage drift (±50 µV/°C) error will be 5.9 LSB
(9.5 mV + 50 PV x 100°C = 1.45 mV {5.9 LSB)
°C
(16)
of the Trigger DAC (U3). The offset voltage related portion of this error can be nulled-out, if necessary, during the
oscilloscope initial calibration. To do so, the LMH6518 input is terminated properly with no input applied and U3
output is adjusted around V_{CM_Aux} voltage (1.2V ±10 mV) while looking for U2’s output transition. U3’s output,
relative to V_{CM_Aux} at transition corresponds to U2’s offset error which can be factored into the Trigger readings
and thus eliminated, leaving only the Offset voltage temperature drift component (= 2 LSB).
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