DESIGN FEATURES
Amplifier with Integrated Filter Offers
the Best High Speed, Low Noise
Interface for Differential
by Michael Kultgen
DACs and ADCs
Introduction
Differential signal paths are becoming a popular way to improve system
performance. A differential signal has
twice the amplitude for a given supply
level. Interference from other components, digital clocks for example,
become common mode signals and
are rejected by the amplifiers in the
differential signal chain. Hence, with a
differential signal path, dynamic range
can be maintained while the supply
voltage is reduced.
Differential output op amps are
one means of providing gain, buffering, and filtering in these signal
paths, but often at the price of added
complexity and board real estate. A
circuit using a differential output
op amp typically requires twice the
number of resistors and capacitors
as the corresponding single-ended
circuit. Therefore, there is a need for
a compact, high performance means
to process differential signals. The
solution is the new LT6600.
The LT6600 is a family of fully differential amplifiers with an integrated 4th
order lowpass filter. Each device in the
family features a fixed cutoff frequency
(2.5MHz, 10MHz and 20MHz) and op10
–10
60
50
–30
40
–50
GROUP DELAY
30
20
–60
10
–70
0
–80
100k
1M
10M
FREQUENCY (Hz)
VMID
7
V+
V–
OUT –
6
5
RF
PROPRIETARY
LOWPASS
FILTER STAGE
11k
V–
+
OP AMP
+ –
–
VOCM
–
VOCM
+
– +
RF
1
VIN–
IN –
RIN
PASSBAND GAIN =
2
3
4
VOCM
V+
OUT +
RF
RIN
WHERE RF =
{
402Ω FOT THE LT6600-10 AND LT6600-20
1580Ω FOR THE LT6600-2.5
Figure 1. LT6600 block diagram
erates with power supplies ranging
from 3V to 10V. The LT6600 is packaged in an SO-8, and is pin-for-pin
compatible with other commercially
available high-speed differential op
amps (Figure 1). Like their industry
counterparts, the LT6600 amplifiers
can accept single-ended or differential
input signals, translate common mode
voltages, and have a common mode
input range that extends to ground.
But, unlike these other amplifiers,
the LT6600s have a proprietary architecture that minimizes noise and
distortion while maximizing speed.
Furthermore, the custom lowpass filter
200Ω
402Ω
43pF
402Ω
VI
287Ω
82pF
402Ω
287Ω
10pF
200Ω
+
OP AMP #1
–
162Ω
82pF
200Ω
162Ω
402Ω
+
OP AMP #2
VO
–
43pF
+
VI
LT6600
402Ω
VO
–
10pF
402Ω
200Ω
100M
Figure 2. The frequency response of the
LT6600-10
22
IN +
11k
70
MAGNITUDE
–20
–40
RIN
8
80
GROUP DELAY (ns)
MAGNITUDE (dB)
0
VIN+
THE COMPETITION
THE BETTER WAY
Figure 3. The LT6600 vs the competition
Linear Technology Magazine • September 2003
DESIGN FEATURES
5V
12pF
SINGLE ENDED
ANALOG INPUT
0.1µF
24.9Ω
1:1
24.9Ω
100Ω
100Ω
12pF
24.9Ω
24.9Ω
V+
+
AIN
–
12pF
LTC174x DOUT
V–
VCM
12pF
Figure 4. Using a transformer to drive a differential ADC
5V
SINGLE ENDED INPUT
REFERENCED TO VCM
+
U1
5V
–
12pF
24.9Ω
24.9Ω
12pF
24.9Ω
100Ω
+
V+
+
AIN
24.9Ω
–
12pF
LTC174x DOUT
V–
VCM
U2
–
4.7µF
499Ω
499Ω
A Great Solution for
Differential ADC and
DAC Interfaces
Figure 5. Using single-ended op amps to drive a differential ADC
DC GAIN =
A
RIN
WHERE A =
{
402Ω FOT THE LT6600-10 AND LT6600-20
1580Ω FOR THE LT6600-2.5
LT6600
RIN
0
800mVP-P
SINGLE ENDED INPUT
REFERENCED TO GND
1
7
0.01µF
t
VIN
RIN
2
8
–
VMID
VOCM
+
5V
5V
3
12pF
+
–
49.9Ω
4
49.9Ω
5
12pF
6
V+
+
12pF
AIN
–
LTC174x DOUT
VCM
0
V–
VCM
1µF
V
VOUT+
VOUT–
3.2VP-P
DIFFERENTIAL SIGNAL
TO THE ADC
t
Figure 6. Using the LT6600 to drive a differential ADC
response provides 30dB attenuation
at 3 times the cutoff frequency with
low delay distortion (Figure 2).
discrete active-RC design would
require 18 precision resistors and
capacitors, as well as a second operational amplifier package (Figure 3).
Quiet, Compact and
The first available devices in the family
Easy to Use
are the LT6600-2.5, the LT6600-10,
The LT6600s are also the most and the LT6600-20, with fixed filter
compact anti-alias/smoothing filter bandwidths of 2.5MHz, 10MHz, and
solutions available. These integrated 20MHz respectively.
filter-amplifiers need only two external
The LT6600-2.5 offers true 14-bit
resistors to set the gain. The precision performance with noise and distortion
response is completely determined by components below –86dB for 1MHz
the monolithic filter. By contrast, a 1VRMS inputs. The LT6600-10 has a
Linear Technology Magazine • September 2003
total noise of 56µVRMS in a 10MHz
bandwidth with harmonics below
–74dB for 5MHz 2VP–P signals. With
the gain set to +12dB, the LT660020 has 42µVRMS total input referred
noise in a 20MHz bandwidth and
shares many features of the -2.5 and
-10. Each member of the family has a
lowpass response with less than 0.5dB
of passband ripple. These combinations of low noise, low distortion and
controlled frequency response are
practically impossible to duplicate
with discrete designs.
The LT6600s are specified and
tested for both single 3V supply
operation and ±5V supply operation.
This flexibility, combined with the low
external parts count and the wide input common mode range, makes the
LT6600 extremely easy to use.
Differential output amplifiers have
gained popularity in systems that
have differential input ADC converters. Often the signal to be processed
by the converter is single-ended, low
amplitude, and from a large source
impedance. For the converter to realize its full range and accuracy, it must
be presented with a larger differential
signal and a common mode level near
mid supply. There are two common
solutions to this problem. Figure 4
shows a transformer coupling circuit
and Figure 5 shows an operational
amplifier circuit. The circuit shown
in Figure 5 is preferable when size
and DC response are important, when
gain is needed, or when buffering is
required. The circuit of Figure 4 is useful for wide bandwidth applications.
Figure 6 shows how simple the differential converter interface is with the
LT6600. This circuit retains all of the
benefits of Figure 5’s circuit (gain, DC
response, and single-ended to differential conversion) with the added feature
of selective anti-alias filtering. Furthermore, the single-ended analog input
can have a common mode level that
differs from the ADC converter (unlike
the circuit of Figure 5). The LT6600
automatically translates the common
23
DESIGN FEATURES
VLT6600 OUT = IDAC •
1+
A
R2 R2
+
R1 R3
WHERE A =
“I”
DIGITAL
INPUT
R3
432Ω
+
–
“Q”
DIGITAL
INPUT
1
7
0.1µF
2
8
R2 402Ω
R3
432Ω
3.3V
3.3V
R3
432Ω
+
R2
402Ω
R1
66.5
–5V
R2 402Ω
R3
432Ω
3.3V
+
4
VCC
LT6600-10
LT5503
3.3V
BI
–5
+
390Ω
8.2pF
2.7nH
0°
90°
MODIN
3.3V
0.1µF
7
0.1µF
3
–
6
1
R1
66.5Ω
LTC1666/7
DIN (Q CHANNEL DAC) IOUT
10mA FS
–
R2
402Ω
R1
66.5
–5V
5V
3.3V
0.1µF
R1
66.5Ω
LTC1666/7
DIN (I CHANNEL DAC) IOUT
10mA FS
402Ω FOT THE LT6600-10 AND LT6600-20
1580Ω FOR THE LT6600-2.5
3.3V
3.3V
5V
{
2
8
MOI
1.2pF
3.3V
+
4
LT6600-10
–5
+
8.2pF
VGA
1.2pF
3
–
18nH
2.7nH
BQ
GND
DMODE
MixEN
ModEN
GC1
GC2
6
Figure 7. Using the LT6600 as a transimpedance amplifier and smoothing filter in a base station application.
mode level when it converts the singleended input to differential. In Figure
6, the input signal is referenced to
ground and the signal presented to
the ADC is referenced to VCM.
To illustrate the excellent dynamic
range of the LT6600, consider Figure 6
with a 1MHz input signal of 800mVPP
amplified by an LT6600-2.5. With RIN
= 402Ω, the amplifier provides 12dB
of voltage gain. The signal presented
to the ADC converter is 3.2VP–P. The
distortion components will be at least
82dB below the fundamental, and the
signal-to-noise ratio will be 81dB in a
5MHz bandwidth.
The differential output DAC is another application where the LT6600
excels. Figure 7 shows the LT6600
acting as a transimpedance amplifier
and a 4th order smoothing filter, in
LT3464, continued from page 21
90
80
VIN = 8.4V
EFFICIENCY (%)
VIN = 4.2V
70
low delay distortion in the passband
(Figure 2), making for an outstanding
DAC smoothing solution.
low current. As shown in Figure 9,
high efficiency is maintained with low
output currents.
For further information on any
of the devices mentioned in this
issue of Linear Technology, use
the reader service card or call the
LTC literature service number:
Conclusion
60
50
40
30
0.01
a base station application. The input
common mode range of the LT6600
accommodates the compliance range
of the DAC. The output common mode
voltage of the LT6600 is set to optimize
the performance of the LT5503 direct
I/Q modulator. The resistors between
the DAC and the LT6600 allow the user
to adjust the transimpedance gain. The
LT6600 and LT5503 are operating on
a 3.3V power supply.
To illustrate the optimized filtering
of the LT6600, consider the case where
the DAC in Figure 7 has a sample rate
of 50Msps and the baseband signal
information extends to 10MHz. By using an LT6600-10, the attenuation of
the images near 40MHz would be more
than 50dB (filter response plus sin(x)/x
attenuation). The excellent rejection
in the stopband is combined with
0.1
1
10
LOAD CURRENT (mA)
100
The LT3464 in the ThinSOT package
produces an ultra compact boost
solution featuring high efficiency,
low quiescent current, true output
disconnect, and low external parts
count.
Conclusion
The LT6600 differential filter-amplifiers are the most compact ADC
anti-aliasing and DAC smoothing
solutions available in the 2.5MHz
to 20MHz range. The combination of
low noise, low distortion, and precision
response are impossible to replicate
with discrete designs. The LT6600 is
pin compatible with standard differential output op amps and performs
all of the same functions. The LT6600
improves the design of any system
requiring differential signal buffering
and filtering.
1-800-4-LINEAR
Ask for the pertinent data sheets
and Application Notes.
Figure 9. Efficiency for the circuit in Figure 7
24
Linear Technology Magazine • September 2003