LMH6550_14 Datasheet, PDF (15/25 Pages) National Semiconductor (TI) – LMH6550 Differential, High Speed Op Amp

LMH6550
www.ti.com
SNOSAK0H – DECEMBER 2004 – REVISED MARCH 2013
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 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 32. Figure 34 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 LMH6550 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
V_CM
37.5:
8 V_PP
R_L = 300:
A_V = 2
ENABLE
Figure 32. Transformer Out High Impedance Load
V_IN * A_V * N
V_L =
§
¨
2
R_OUT
*
N²
¨©
R_L
+1
WHERE V_IN = DIFFERENTIAL INPUT VOLTAGE
N = TRANSFORMER TURNS RATIO =
¨§ ^SECONDARY
¨
©
PRIMARY
A_V = CLOSED LOOP AMPLIFIER GAIN
R_OUT = SERIES OUTPUT MATCHING RESISTOR
R_L = LOAD RESISTOR
V_L = VOLTAGE ACROSS LOAD RESISTOR
Figure 33. Calculating Transformer Circuit Net Gain
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