LMH6518SQEVK National Semiconductor, LMH6518SQEVK Datasheet - Page 17

• Manufacturer Part Number: LMH6518SQEVK
• Description: KIT EVAL FOR LMH6518 VGA
• Manufacturer: National Semiconductor
• Series: PowerWise®r

Specifications of LMH6518SQEVK

• Channels Per Ic: 1 - Single
• Amplifier Type: Variable Gain
• Output Type: Differential
• -3db Bandwidth: 900MHz
• Operating Temperature: -40°C ~ 85°C
• Current - Supply (main Ic): 210mA
• Voltage - Supply, Single/dual (±): 4.75 V ~ 5.25 V
• Board Type: Fully Populated
• Utilized Ic / Part: LMH6518
• Lead Free Status / RoHS Status: Lead free / RoHS Compliant
• Current - Output / Channel: -
• Slew Rate: -
• Lead Free Status / Rohs Status: Supplier Unconfirmed

Applications Information
FUNCTIONAL DESCRIPTION AND DYNAMIC RANGE IN
OSCILLOSCOPE APPLICATIONS
Here is a block diagram of the LMH6518’s Main Output signal
path:
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 adjust-
ment range (from −1.16 dB to 38.8 dB) allows this device to
be used with the National 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
FS input range. The National Semiconductor GSample/sec-
ond 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:
Overdrive Recovery Time (Return to Zero)
FIGURE 1. LMH6518 Signal Path Block Diagram
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The recommended ADC FS operating range is, however,
narrower and it is from 595 mV to 805 mV with 700 mV
the mid-point. Raising the value of ADC FS voltage is tanta-
mount 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
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 accom-
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