LMH6555_14 Datasheet, PDF (23/37 Pages) Texas Instruments – LMH6555 Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011
DATA ACQUISITION APPLICATIONS
Figure 39 shows the LMH6555 used as the differential driver to the Texas Instruments ADC081500 running at
1.5G samples/second.
R_S1
340 mV_{PP 50:}
a ^VIN
R_S2
50:
R_F1
V_IN+ ^RG1
+
-
V_IN- _RG2
LMH6555 ^RF2
R_T2
50:
R_T1
50:
V_OUT = 0.8 V_PP
V_OUT+
ADC081000/
ADC081500
V_{CMO SPI}
V_{CM_REF}
3.3V
+
OPT
LMV321 _OPT
-
Figure 39. Schematic of the LMH6555 Interfaced to the ADC081500
In the schematic of Figure 39, the LMH6555 converts a single ended input into a differential output for direct
interface to the ADC's 100Ω differential input. An alternative approach to using the LMH6555 for this purpose,
would have been to use a balun transformer, as shown in Figure 40.
R_S1
1.6 V_PP ^50:
V_IN
R_S2
50:
6
4.7 nF
1
4
3 ^{4.7 nF}
MINI CIRCUITS
TYPE
TCI-1-13M
TO ADC
V_IN+
800 mV_PP
TO ADC
V_IN-
Figure 40. Single Ended to Differential Conversion
(AC only) with a Balun Transformer
In the circuit of Figure 40, the ADC will see a 100Ω differential driver which will swing the required 800 mV_PP
when V_IN is 1.6 V_PP. The source (V_IN) will see an overall impedance of 200Ω for the frequency range that the
transformer is specified to operate. Note that with this scheme, the signal to the ADC must be AC coupled,
because of the transformer’s minimum operating frequency which would prevent DC coupling. For the
transformer specified, the lower operating frequency is around 4.5 MHz and the input high pass filter’s −3 dB
bandwidth is around 340 kHz for the values shown (or (1/πR_EQC)Hz where R_EQ = 200Ω).
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