LMH6401_16 Datasheet, PDF (31/48 Pages) Texas Instruments – LMH6401 DC to 4.5 GHz, Fully-Differential, Digital Variable-Gain Amplifier

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LMH6401
SBOS730A – APRIL 2015 – REVISED MAY 2015
Typical Application (continued)
10.2.1 Design Requirements
Table 12 shows example design requirements for an amplifier in an oscilloscope front-end application; the
LMH6401 meets these requirements.
Table 12. Example Design Requirements for an Oscilloscope Front-End application
SPECIFICATION
Supply voltage and current
Usable input frequency range
Voltage gain and gain range
OIP3 (P_O= –2 dBm per tone, R_L= 200 Ω) and noise figure (R_S = 100 Ω) at 1 GHz
Rise and fall time (V_O= 2-V step) from 10% to 90%
Settling time to 1% of V_O = 2-V step
DESIGN REQUIREMENTS
4.0 V to 5.25 V with typically less than a 100-mA current and
split-supply operation supported
DC to 2 GHz
26-dB to 10-dB voltage gain with x2 attenuation supported
(ideal 32-dB gain range)
> 30 dBm and less than 10 dB, respectively.
Less than 100 ps
Less than 1 ns with long-term settling correction required
10.2.2 Detailed Design Procedure
10.2.2.1 Driving ADCs
When the amplifier is driving an ADC, the key points to consider for implementation are the signal-to-noise ratio
(SNR), spurious-free dynamic range (SFDR), and ADC input considerations, as described in this section.
A typical application of the LMH6401 involves driving an ultra-wideband, 12-bit ADC (such as the ADC12J4000),
as shown in Figure 66. The LMH6401 can drive the full Nyquist bandwidth of ADCs with sampling rates up to
4 GSPS. If the front-end bandwidth of the ADC is more than 2 GHz, use a simple noise filter to improve SNR.
Otherwise, the ADC can be connected directly to the amplifier output pins with appropriate matching resistors to
limit the full-scale input of the ADC. Note that the LMH6401 inputs must be driven differentially using a balun or
fully-differential amplifiers (FDAs). For dc-coupled applications, an FDA (such as the LMH3401 or LMH5401) that
can convert a single-ended input to a differential output with low distortion is preferred.
LMH5401
VS+ = 3.73 V
VS+ = 3.73 V
1.23 V
RF
25 Ÿ
IN
RT
RG
10 Ÿ
0 Ÿ
-
+
+-
10 Ÿ
RO+ = 40 Ÿ
LMH6401
RM
RG
RF
25 Ÿ
10 Ÿ
0 Ÿ
~ 100 Ÿ
10 Ÿ
VOCM ^{RO- = 40 Ÿ}
VS- = -1.27 V
VS- = -1.27 V
1.5 pF (2) 95 Ÿ
A^AD^DS^C4¹1²B^J4⁴9⁰⁰⁰
VOCM
0.01 2F
Figure 66. DC-Coupled Oscilloscope Front-End Application
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