LMH6551 Datasheet, PDF (15/32 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

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LMH6551
SNOSAK7D – FEBRUARY 2005 – REVISED JANUARY 2015
Typical Applications (continued)
The circuit shown in Figure 23 is a typical fully differential application as might be used to drive an ADC. In this
circuit closed loop gain, (A_V) = V_OUT/ V_IN = R_F/R_G. For all the applications in this data sheet V_IN is presumed to be
the voltage presented to the circuit by the signal source. For differential signals this will be the difference of the
signals on each input (which will be double the magnitude of each individual signal), while in single-ended inputs
it will just be the driven input signal.
The resistors R_O help keep the amplifier stable when presented with a load C_L as is typical in an analog to digital
converter (ADC). When fed with a differential signal, the LMH6551 provides excellent distortion, balance and
common-mode rejection provided the resistors R_F, R_G and R_O are well matched and strict symmetry is observed
in board layout. With a DC CMRR of over 80dB, the DC and low frequency CMRR of most circuits will be
dominated by the external resistors and board trace resistance. At higher frequencies board layout symmetry
becomes a factor as well. Precision resistors of at least 0.1% accuracy are recommended and careful board
layout will also be required.
500
50:
100:
TWISTED PAIR
250
+
a ^{2 V}PP
V_CM
-
250
GAIN = 2
2 V_PP
50:
500
Figure 24. Fully Differential Cable Driver
With up to 15 V_PP differential output voltage swing and 80 mA of linear drive current the LMH6551 makes an
excellent cable driver as shown in Figure 24. The LMH6551 is also suitable for driving differential cables from a
single-ended source.
The LMH6551 requires supply bypassing capacitors as shown in Figure 25 and Figure 26. The 0.01 µF and 0.1
µF capacitors should be leadless SMT ceramic capacitors and should be no more than 3 mm from the supply
pins. The SMT capacitors should be connected directly to a ground plane. Thin traces or small vias will reduce
the effectiveness of bypass capacitors. Also shown in both figures is a capacitor from the V_CM pin to ground. The
V_CM pin is a high impedance input to a buffer which sets the output common-mode voltage. Any noise on this
input is transferred directly to the output. Output common-mode noise will result in loss of dynamic range,
degraded CMRR, degraded Balance and higher distortion. The V_CM pin should be bypassed even if the pin in not
used. There is an internal resistive divider on chip to set the output common-mode voltage to the mid point of the
supply pins. The impedance looking into this pin is approximately 25 kΩ. If a different output common-mode
voltage is desired drive this pin with a clean, accurate voltage reference.
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