LMH2832 Datasheet, PDF (33/46 Pages) Texas Instruments – LMH2832 Fully Differential, Dual, 1.1-GHz, Digital Variable-Gain Amplifier

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LMH2832
SBOS709A – JULY 2016 – REVISED JULY 2016
10.2.1.2 Detailed Design Procedure
To begin the design process, make sure that none of the following design parameters exceed the limits listed in
the Electrical Characteristics table, such as:
• Supply voltage
• Temperature range
• Input voltage range across gain
• Output current requirements
• Digital I/O voltages and currents
10.2.1.2.1 Source Resistance Matching
Standard DOCSIS systems use a characteristic single-ended impedance of 75 Ω that must be properly matched
to the 150-Ω differential impedance of the LMH2832. The circuit in Figure 61 uses a transformer with a 1:2-Ω
ratio to convert the signal from single-ended to differential, and also to match the differential impedance. The
transformer also adds a signal gain of approximately 3 dB to the system with some insertion loss depending on
the chosen transformer.
10.2.1.2.2 Output Impedance Matching
For the circuit in Figure 61, the output impedance is matched to a 150-Ω characteristic impedance filter to
maximize the performance of the LMH2832. On the amplifier output side, the output impedance is matched to
150 Ω by including a 65-Ω series resistor on each output. Combined with the internal 10-Ω resistors on each
output, the total differential impedance becomes 150 Ω. The ADS54J40 has an input impedance of
approximately 600 Ω that is reduced to 150 Ω by using two 5-Ω series input resistors in parallel with two 100-Ω
series resistors. The 5-Ω series resistors are included to isolate the input capacitance of the ADC so that the
response of the filter is not affected. With both the amplifier and ADC impedances matched, any transmission
line effects of the connection are minimized.
If the ADC is physically located close enough to the amplifier, a matched impedance may not be needed; see
Driving Low Insertion-Loss Filters section for more information on driving non-matched filters.
10.2.1.2.3 Voltage Headroom Considerations
Because of the series resistors included on both the amplifier outputs and ADC inputs, the amplifier must drive a
voltage that is significantly higher than the ADC full-scale input. For the circuit in Figure 61, the ADS54J40 full-
scale input voltage is 1.9 V_PP, so the required voltage at the amplifier output pins is 3.6 V_PP. This voltage is less
than the specified output voltage of 5 V_PP for the LMH2832, thus system performance is not limited. If the
required output voltage is higher than what the amplifier can support, then the matched resistance value can be
reduced. However, this reduction can have performance implications because more current output is required
from the amplifier.
The input voltage swing can be larger than the output voltage swing because the LMH2832 can operate as an
attenuator. To maintain the full-scale voltage of the ADS54J40 input in this application, the amplifier cannot
attenuate more than 1 dB from input to output; otherwise, the maximum input voltage swing is exceeded. If the
amplifier must be operated with more attenuation, then the output voltage must be reduced.
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