LMH6554_15 Datasheet, PDF (18/29 Pages) Texas Instruments – LMH6554 2.8-GHz Ultra Linear Fully Differential Amplifier

LMH6554
SNOSB30P – OCTOBER 2008 – REVISED JANUARY 2015
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9.2.4 Driving Analog-to-Digital Converters
Analog-to-digital converters present challenging load conditions. They typically have high impedance inputs with
large and often variable capacitive components. Figure 32 shows the LMH6554 driving an ultra-high-speed
Gigasample ADC the ADC10D1500. The LMH6554 common mode voltage is set by the ADC10D1500. The
circuit in Figure 32 has a 2nd order bandpass LC filter across the differential inputs of the ADC10D1500. The
ADC10D1500 is a dual channel 10–bit ADC with maximum sampling rate of 3 GSPS when operating in a single
channel mode and 1.5 GSPS in dual channel mode.
R_S = 50:
91: 200:
V_S
a
C
AC-Coupled
Source
91:
30:
76.8:
0.1 PF _VCM
V⁺
+-
LMH6554
-+
V^-
0.1 PF
0.1 PF
200:
50:
50:
V_EN
ADC
Up To 16-Bit
Data Converter
V_CMO
Figure 32. Driving a 10-bit Gigasample ADC
Figure 33 shows the SFDR and SNR performance vs. frequency for the LMH6554 and ADC10D1500
combination circuit with the ADC input signal level at −1dBFS. In order to properly match the input impedance
seen at the LMH6554 amplifier inputs, R_M is chosen to match Z_S || R_T for proper input balance. The amplifier is
configured to provide a gain of 2 V/V in single to differential mode. An external bandpass filter is inserted in
series between the input signal source and the amplifier to reduce harmonics and noise from the signal
generator.
90
85
80
75
SFDR (dBm)
70
65
60
55
50
SNR (dBFs)
45
40
0 100 200 300 400 500 600 700 750
INPUT FREQUENCY (MHz)
Figure 33. LMH6554 / ADC10D1500 SFDR and SNR Performance vs. Frequency
The amplifier and ADC should be located as close together as possible. Both devices require that the filter
components be in close proximity to them. The amplifier needs to have minimal parasitic loading on it's outputs
and the ADC is sensitive to high frequency noise that may couple in on its inputs. Some high performance ADCs
have an input stage that has a bandwidth of several times its sample rate. The sampling process results in all
input signals presented to the input stage mixing down into the first Nyquist zone (DC to Fs/2).
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