LMH6518_15 Datasheet, PDF (19/41 Pages) Texas Instruments – LMH6518 900 MHz, Digitally Controlled, Variable Gain Amplifier

LMH6518
www.ti.com
APPLICATIONS INFORMATION
SNOSB21C – MAY 2008 – REVISED JULY 2013
FUNCTIONAL DESCRIPTION AND DYNAMIC RANGE IN OSCILLOSCOPE APPLICATIONS
Here is a block diagram of the LMH6518’s Main Output signal path:
+In
Pre-amp
10 dB or 30 dB
Ladder Attenuator
10 Steps, 2 dB/ Step
0 to -20 dB
Output
Amp
8.86 dB
-In
50:
50:
+Out
-Out
Figure 57. LMH6518 Signal Path Block Diagram
The Auxiliary output (not shown) uses another but similar Output Amp that taps into the Ladder Attenuator
output. In this document, Preamp gain of 30 dB is referred to as “Preamp HG” (High Gain) and Preamp gain of
10 dB as “Preamp LG” (Low Gain).
The LMH6518’s 2 dB/step gain resolution and 40 dB adjustment range (from −1.16 dB to 38.8 dB) allows this
device to be used with the TI GSample/second ADCs which have Full Scale, FS, adjustment (through their
Extended Control Mode or ECM) to provide near-continuous variability (8.5 mdB resolution) to cover a 42.6 dB
920 mV_PP
(20 x log _{6.8 mVPP = 42.6 dB)}
(1)
FS input range. The Texas Instruments GSample/second ECM control allows the ADC FS to be set using the
ADC SPI bus. The ADC FS voltage range is from 560 mV to 840 mV with 9 bits of FS voltage control.
The ADC ECM gain resolution can be calculated as follows:
0.56 +
§
¨
0.84
±
0.56
¨
©
2 x 512
Gain Resolution = 20 log
= 8.5 mdB
0.56
±
§
¨
¨
©
0.84 ± 0.56
2 x 512
(2)
The recommended ADC FS operating range is, however, narrower and it is from 595 mV to 805 mV with 700
mV_PP as the mid-point. Raising the value of ADC FS voltage is tantamount to reducing the signal path gain to
accommodate a larger input and vice versa, thus providing a method of gain fine-adjust. The ADC ECM gain
adjustment is −1.21 dB
(= 20 x log ^{700 mV} ) to +1.41 dB
805 mV
(= 20 x log ^{700 mV} )
595 mV
(3)
Because the ADC FS fine-adjust range of 2.62 dB (= 1.41 dB + 1.21 dB) is larger than the LMH6518’s 2 dB/step
resolution, there is always at least one LMH6518 gain setting to accommodate any FS signal from 6.8 mV_PP to
920 mV_PP, at the LMH6518 input, with 0.62 dB (= 2.62-2) overlap.
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