LMH6555_14 Datasheet, PDF (11/37 Pages) Texas Instruments – LMH6555 Low Distortion 1.2 GHz Differential Driver

LMH6555
www.ti.com
SNOSAJ1C – NOVEMBER 2006 – REVISED MARCH 2011
APPLICATION INFORMATION
See DEFINITION OF TERMS AND SPECIFICATIONS (ALPHABETICAL ORDER) for definition of terms used.
GENERAL
The LMH6555 consists of three individual amplifiers:
1. V_OUT+ driver
2. V_OUT^− driver
3. The common mode amplifier
Being a differential amplifier, the LMH6555 will not respond to the common mode input (as long as it is within its
input common mode range) and instead the output common mode is forced by the built-in common mode
amplifier with V_{CM_REF} as its input. As shown, in Figure 23 below, the V_CMO output of most differential high speed
ADC’s is tied to the V_{CM_REF} input of the LMH6555 for direct output common mode control. In some cases, the
output drive capability of the ADC V_CMO output may need an external buffer, as shown, to increase its current
capability in order to drive the V_{CM_REF} pin. The Electrical Characteristics Table shows the gain (Gain_V_{CM_REF})
and the offset (V_{OS_CM}) from the V_{CM_REF} to the device output common mode.
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 23. Single Ended to Differential Conversion
The single ended input and output impedances of the LMH6555 I/O pins are close to 50Ω as specified in
Electrical Characteristics Table (R_IN and R_O). With differential input drive, the differential input impedance
(R_{IN_DIFF}) is close to 78Ω.
The device nominal input common mode voltage (V_{I_CM}) is close to 0.3V when R_S1 and R_S2 of Figure 23 are
open. Thus, the input source will experience a DC current with 0V input. Because of this, the differential output
offset voltage is influenced by the matching between R_S1 and R_S2. So, in a single ended input condition, if the
signal source is AC coupled to one input, the undriven input needs to also be AC coupled in order to cancel the
output offset voltage (V_OOS).
In applications where low output offset is required, it is possible to inject some current to the appropriate input
(V_IN+ or V_IN^−) as an effective method of trimming the output offset voltage of the LMH6555. This is explained later
in this document. The nominal value of R_S1 and R_S2 will also affect the insertion gain (A_{V_DIFF}). The LMH6555 can
also be used with the input AC coupled through equal valued DC blocking capacitors (C) in series with V_IN+ and
V_IN^−. In this case, the coupling capacitors need to be large enough to not block the low frequency content. The
lower cutoff frequency will be 1/(πR_EQC)Hz with R_EQ= R_S1+ R_S2 + R_{IN_DIFF} where R_{IN_DIFF} ≈ 78Ω.
The single ended output impedance of the LMH6555 is 50Ω. The LMH6555 Electrical Characteristics shows the
device performance with 100Ω differential output load, as would be the case if a device such as the ADC081000/
ADC081500 (single/ dual ADC) were being driven.
Copyright © 2006–2011, Texas Instruments Incorporated
Product Folder Links: LMH6555
Submit Documentation Feedback
11