LMH6550 Datasheet, PDF (15/33 Pages) National Semiconductor (TI) – Differential, High Speed Op Amp

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LMH6550
SNOSAK0I – DECEMBER 2004 – REVISED JANUARY 2015
Typical Applications (continued)
9.2.1.2 Detailed Design Procedure
The power supplies for this design are symmetrical ±5-V supplies (not shown for simplicity). The ADC input
common mode is 1 V which is within the optimum operating range for the LMH6550 when used on ±5-V split
supplies. The gain of this circuit is equal to RF/RG and due to the split supplies can be set to gains of 15 V/V or
less. Higher gains will result in values of RF that are too large for high speed operation.
9.2.1.2.1 Fully Differential Operation
The circuit shown in 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 LMH6550 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 80 dB, 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
500
2 V_PP
50:
GAIN = 2
ENABLE
Figure 22. Fully Differential Cable Driver
With up to 15 V_PP differential output voltage swing and 80 mA of linear drive current the LMH6550 makes an
excellent cable driver as shown in Figure 22. The LMH6550 is also suitable for driving differential cables from a
single-ended source.
The LMH6550 requires supply bypassing capacitors as shown in Figure 23 and Figure 24. 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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