LMH6551Q_15 Datasheet, PDF (16/26 Pages) Texas Instruments – Differential, High Speed Op Amp

LMH6551Q
SNOSB95E – NOVEMBER 2011 – REVISED MARCH 2013
R_G1
a ^VI
R_G2
R_F1
56
+
V_CM
-
39 pF
39 pF
56
R_F2
ADC12LO66
7 - 8 pF
V_REF
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1V LOW IMPEDANCE
VOLTAGE REFERENCE
Figure 32. Driving an ADC
The amplifier and ADC should be located as closely 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 the output
traces and the ADC is sensitive to high frequency noise that may couple in on its input lines. 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 Nyquist range (DC to Fs/2).
See AN-236 (SNAA079) for more details on the subsampling process and the requirements this imposes on the
filtering necessary in your system.
USING TRANSFORMERS
Transformers are useful for impedance transformation as well as for single to differential, and differential to single
ended conversion. A transformer can be used to step up the output voltage of the amplifier to drive very high
impedance loads as shown in Figure 33. Figure 35 shows the opposite case where the output voltage is stepped
down to drive a low impedance load.
Transformers have limitations that must be considered before choosing to use one. Compared to a differential
amplifier, the most serious limitations of a transformer are the inability to pass DC and balance error (which
causes distortion and gain errors). For most applications the LMH6551Q will have adequate output swing and
drive current and a transformer will not be desirable. Transformers are used primarily to interface differential
circuits to 50Ω single ended test equipment to simplify diagnostic testing.
300: TWISTED PAIR
500
37.5: 1:2 (TURNS)
250
+
a 4 VPP
V_CM
-
250
500
A_V = 2
V_CM
37.5:
8 V_PP
R_L = 300:
Figure 33. Transformer Out High Impedance Load
16
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