Max1195, Using transformer coupling, Single-ended ac-coupled input signal – Rainbow Electronics MAX1195 User Manual
Page 16: Buffered external reference drives multiple adcs
MAX1195
Using Transformer Coupling
An RF transformer (Figure 6) provides an excellent
solution to convert a single-ended source signal to a
fully differential signal, required by the MAX1195 for
optimum performance. Connecting the center tap of the
transformer to COM provides a V
DD
/2 DC level shift to
the input. Although a 1:1 transformer is shown, a step-
up transformer can be selected to reduce the drive
requirements. A reduced signal swing from the input
driver, such as an op amp, can also improve the overall
distortion.
In general, the MAX1195 provides better SFDR and
THD with fully differential input signals than single-
ended drive, especially for very high input frequencies.
In differential input mode, even-order harmonics are
lower as both inputs (INA+, INA- and/or INB+, INB-) are
balanced, and each of the ADC inputs only requires
half the signal swing compared to single-ended mode.
Single-Ended AC-Coupled Input Signal
Figure 7 shows an AC-coupled, single-ended applica-
tion. Amplifiers like the MAX4108 provide high speed,
high bandwidth, low noise, and low distortion to main-
tain the integrity of the input signal.
Buffered External Reference Drives
Multiple ADCs
Multiple-converter systems based on the MAX1195 are
well suited for use with a common reference voltage.
The REFIN pin of those converters can be connected
directly to an external reference source.
A precision bandgap reference like the MAX6062 gen-
erates an external DC level of 2.048V (Figure 8), and
exhibits a noise voltage density of 150nV/
√Hz. Its out-
put passes through a 1-pole lowpass filter (with 10Hz
cutoff frequency) to the MAX4250, which buffers the
reference before its output is applied to a second 10Hz
lowpass filter. The MAX4250 provides a low offset volt-
age (for high gain accuracy) and a low noise level. The
Dual, 8-Bit, 40Msps, 3V, Low-Power ADC with
Internal Reference and Parallel Outputs
16
______________________________________________________________________________________
MAX1195
T1
N.C.
V
IN
6
1
5
2
4
3
22pF
22pF
0.1
µF
0.1
µF
2.2
µF
25
Ω
25
Ω
MINICIRCUITS
TT1–6-KK81
T1
N.C.
V
IN
6
1
5
2
4
3
22pF
22pF
0.1
µF
0.1
µF
2.2
µF
25
Ω
25
Ω
MINICIRCUITS
TT1–6-KK81
INA-
INA+
INB-
INB+
COM
MAX1195
0.1
µF
1k
Ω
1k
Ω
100
Ω
100
Ω
C
IN
22pF
C
IN
22pF
INB+
INB-
COM
INA+
INA-
0.1
µF
R
ISO
50
Ω
R
ISO
50
Ω
REFP
REFN
V
IN
MAX4108
0.1
µF
1k
Ω
1k
Ω
100
Ω
100
Ω
C
IN
22pF
C
IN
22pF
0.1
µF
R
ISO
50
Ω
R
ISO
50
Ω
REFP
REFN
V
IN
MAX4108
Figure 6. Transformer-Coupled Input Drive
Figure 7. Using an Op Amp for Single-Ended, AC-Coupled
Input Drive