LMH2832 Datasheet, PDF (20/46 Pages) Texas Instruments – LMH2832 Fully Differential, Dual, 1.1-GHz, Digital Variable-Gain Amplifier

LMH2832
SBOS709A – JULY 2016 – REVISED JULY 2016
www.ti.com
Feature Description (continued)
At maximum gain, the digital attenuator is set to 0 dB of attenuation, causing the input signal to be much larger
than the output signal and forcing the maximum output voltage swing to be limited by the outputs of the device.
However at minimum gain, the maximum voltage swing is limited by the inputs of the device because the
attenuator causes the output voltage to be 9 dB lower than the input voltage. In minimum gain, the input voltage
limits against the electrostatic discharge (ESD) devices before the output reaches the maximum swing limits. For
linear operation, the input voltage must be kept within the maximum input voltage ratings described in the
Electrical Characteristics table.
The input impedance of the LMH2832 is set by internal termination resistors to a nominal value of 150 Ω,
differential. Process variations result in a range of values, as described in the Electrical Characteristics table. The
input impedance is also affected by device parasitic effects at higher frequencies that cause the impedance to
shift away from the nominal value.
9.3.2 Analog Output Characteristics
The LMH2832 has series, 10-Ω, on-chip resistors on each output to provide isolation from board parasitics that
can cause instability. When designing a filter following the LMH2832, the filter source impedance calculation
must include the two 10-Ω, on-chip resistors. Table 2 shows the calculated external source impedance values
required for various matched filter loads.
Table 2. Output Resistor Values for Matched Loads
MATCHED FILTER IMPEDANCE (Ω)
100
150
200
300
EXTERNAL SERIES RESISTOR PER OUTPUT (Ω)
80
130
180
280
9.3.3 Driving Low Insertion-Loss Filters
When driving high-speed ADCs, a filter is commonly driven with a matched impedance to the ADC. This
impedance is matched by the amplifier by setting the combination of the output resistors to the same impedance
as the ADC inputs. Impedance matching is often done to minimize any transmission reflections caused by the
physical signal path. The drawback to using a matched impedance is that a voltage swing is required from the
amplifier outputs that is twice the desired ADC input voltage swing, which can cause output voltage limitation
issues.
To avoid using a matched impedance, a low insertion loss filter can be driven where there is little to no
resistance added at the amplifier outputs. The amplifier outputs then only must swing to the value of the ADC
full-scale input voltage, thus eliminating most of the potential amplifier output headroom issues. The requirements
of this technique are that the amplifier must be able to provide enough current to the load of the ADC and that
the path between the amplifier outputs and ADC inputs must be minimized to prevent any reflections.
9.3.4 Input Impedance Matching
The LMH2832 has a differential input impedance of 150 Ω that can be easily matched to single-ended, 50-Ω or
75-Ω systems using baluns. For a single-ended, 50-Ω input, a 1:3-Ω ratio balun can be used to create a 150-Ω
differential source impedance to the device with a balun gain of 4.8 dB. For a single-ended, 75-Ω input, a 1:2-Ω
ratio balun creates a 150-Ω differential source impedance to the device with a voltage gain of 3 dB.
9.3.5 Power-On Reset (POR)
The LMH2832 has a built-in, power-on-reset (POR) that sets the device registers to their default state (see the
Register Maps section) on power-up. Note that the LMH2832 register information is lost when power is removed.
When power is reapplied, the POR ensures that the device enters a default state. Power glitches (of sufficient
duration) can also initiate the POR and return the device to a default state.
20
Submit Documentation Feedback
Product Folder Links: LMH2832
Copyright © 2016, Texas Instruments Incorporated