DN454 - Single-Ended to Differential Amplifier Design Tips

Single-Ended to Differential Amplifier Design Tips – Design Note 454
Philip Karantzalis and Tim Regan
Introduction
A fully differential amplifier is often used to convert
a single-ended signal to a differential signal, a design
which requires three significant considerations: the
impedance of the single-ended source must match the
single-ended impedance of the differential amplifier,
the amplifier’s inputs must remain within the common
mode voltage limits and the input signal must be level
shifted to a signal that is centered at the desired output
common mode voltage.
In all cases, input impedance matching to the source
impedance is necessary to prevent high frequency
reflections. In designs where the single-ended source
is DC coupled to a single supply differential amplifier,
then level shifting and the common mode limits are
also important considerations. The interaction of these
three design parameters is non-trivial—component
selection requires spreadsheet analysis using the
equations described here.
Input Impedance Matching
If input AC coupling is used, then impedance matching
is the only design issue. Figure 1 shows an example
of a circuit matching a 50Ω single-ended source to an
AC-coupled LTC ®6400-20 differential amplifier with a
gain of 20dB set by internal resistors.
The 66.5Ω resistor, RT, in parallel with the +IN input
impedance, ZIN, matches the circuit input impedance
to the 50Ω source. Differential balance is provided
with the addition of the 28.7Ω resistor at the –IN
input, R2. The balancing resistor assures equivalent
feedback factors at the inputs, thus preventing large
DC offsets.
To calculate the external resistor values, start by calculating ZIN. Then calculate RT for impedance matching and
the value of the R2 for differential balance. The overall
single-ended to differential gain (GAIN) must take into
account the input attenuation of the RS and RT resistive
divider and the effect of adding R2. In this example,
11/08/454
ZIN
RS
50Ω
VIN
R1
100Ω
0.1μF
LTC6400-20
12.5Ω
RF
1000Ω
13 +IN
+
–
+OUT 8
50Ω
RT
66.5Ω
IN+
14 +IN
+ –
_
15 –IN
IN–
+
OUT–
+OUTF 7
1.7pF
50Ω
–OUTF 6
OUT+
RF
1000Ω
R1
100Ω
0.1μF
12.5Ω
16 –IN
R2
28.7Ω
–OUT 5
VOUT–
DN454 F01
RF2 • 4 • R12 RS2 8 • RF • R13 4 • R14 RF • 2 • R1 RS 2 • R1• R1 RS
ZIN RT VOUT+
2 • RF 2 • R1 RS
RS • ZIN
ZIN – RS
GAIN R2 RS • R T
RS R T
–
R1 R2 RF • R1• RS – R T RS • R T
VOUT
– VOUT
VIN
RS • R T • R1 R2
Figure 1. Impedance Matching for a Differential
Amplifier with Fixed Gain Integrated Resistors
the overall gain of the amplifier from signal source to
differential output is only 4.44 even though the amplifier
has a fixed gain of 10.
By AC coupling at the input, the amplifier’s input common mode voltage is equal to its output common mode
voltage and the single-ended signal is automatically
level shifted to an output differential signal centered
on the output common mode voltage.
If the input common mode voltage is not 0V, and the
source cannot deliver the DC current into 116.5Ω (50Ω
+ 66.5Ω), then it is also necessary to AC couple the
66.5Ω resistor.
The DC Coupled Differential Amplifier
A general purpose, DC coupled, single-ended-to-differential amplifier circuit with source impedance matching and input level shifting is shown in Figure 2. Level
shifting is provided by the reference voltage (VREF ).
If VREF is set to be equal to the input common mode
voltage (VINCM) then the single-ended input signal is
L, LT, LTC and LTM are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
Figure 3 shows an example of a single-ended-to-differential amplifier matching a 75Ω source and level
shifting from a 2.5V input common mode to a 1.25V
output common mode voltage (typical level shifting
required from a 5V single-ended circuit to a 3V differential circuit to drive a high speed ADC). The singleended-to-differential gain of the Figure 3 amplifier
is 2 (the 1VP-P input signal is amplified into a 2VP-P
differential output signal, a typical input voltage range
of a high speed ADC).
RF
VINCM
VINP-P
VIN
V+
RS VT
+
–
R1
RT
VOCM
R2
VA
VA
R1
– +
VOCM
VOUTP-P
+ –
VOCM
VOUTP-P
VREF
GAIN =
VREF = VINCM
RF
DN454 F02
2 • VOUTP-P
= GN
VINP-P
GN2 • 4 • R12 RS2 2 • GN • R1• 2 • R1– RS R12 GN • 2 • R1 RS – R1
RF RT RS • R1 RF
R1 RF – RS • GN 1
2
RS • R T
R2 RS R T
For linear operation, the amplifier’s input common
mode limits must not be exceeded. Figure 2 shows
the calculations for the bias voltage (VT) of the input
R1 RF • RS RT • VINCM RS • RT • VOCM p VINP-P
VT T-network
(RS, RT and R1) and the common mode volt4
R1• RS R T RF • RS R T RS • R T
age at the differential amplifier’s inputs. For example, in
Figure 2. Impedance Matching and Level Shifting for a
Figure 3, the 1.99V to 2.44V at the amplifier’s inputs (as
Differential Amplifer with Gain Set By External Resistors calculated by the V equation) is well within the rail-to-rail
A
+
shifted to a differential signal centered on the output input common mode range of the LTC6406 (0V to V ).
common mode voltage (VOCM).
Table 1. Sample of LTC High Speed Differential Amplifiers
VA ¥
´
GN
RF • VINCM ¦ VOCM p
• VINP-P µ • R1 R2
§
¶
2
R1 R2 RF
The design of a single-ended to differential amplifier
with external resistors provides an additional design
option: specifying the amplifier gain. Figure 2 shows
the design equations when the RF and R1 resistors are
selectable, not fixed.
AMPLIFIER
LTC6400-26
LTC6400-20
LTC6400-14
LTC6400-8
LTC6401-20
LTC6401-14
LTC6404-1
LTC6404-2
LTC6405
LTC6406
The design of this circuit begins with the value of R1.
This resistor must be larger than the input source
resistance but not so large as to increase circuit noise.
Next, calculate the value of the feedback resistor RF
using the desired gain (GN). Then calculate the value
of resistors RT and R2.
2.5V
VIN
SLEW
RATE V/μs
6670
4500
4800
3810
4500
3600
450
700
690
630
VOLTAGE
NOISE
nV/√Hz
1.5
2.1
2.5
3.7
2.1
2.5
1.5
1.5
1.6
1.6
GAIN
V/V
20
10
5
2.5
10
5
R SET
R SET
R SET
R SET
1.8pF
1VP-P
+
–
GBW
GHz
1.9
1.8
1.9
2.2
1.3
2
0.5
0.9
2.7
3
75Ω
150Ω
665Ω
3V
0.1μF
102Ω
1.25V
VOCM
– +
+ –
®
3V
LT 6660-2.5
IN
OUT
GND
0.1μF
2.5V
0.1μF
43.2Ω 150Ω
10μF
1.25V
1VP-P
1.25V
1VP-P
LTC6406
665Ω
GAIN = 2
1.8pF
DN454 F03
Figure 3. Putting it All Together: A 133MHz Differential Amplifier with External Gain Setting
Resistors,Impedance Matching to a 75Ω Source and Shifting from 2.5V to 1.25V
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