ISL55210_14 Datasheet, PDF (15/18 Pages) Intersil Corporation – Wideband, Low-Power, Ultra-High Dynamic Range Differential Amplifier

ISL55210
any V_CM level shifting that needs to take place, the filter shape,
and this I_CM issue into the ADC input pins. Here are 4 example
topologies suitable for different situations.
1. AC coupled, broadband RLC interstage filter design. This
approach lets the amplifier operate at its desired output
common mode, then provides the ADC common mode
voltage and current through a bias path as part of the filter
design’s last stage R values. The V_B is set to include the IR
loss from that voltage to the ADC inputs due to the I_CM
current. This circuit is the one shown on the front page where
we get a usable frequency range from about 500kHz to
150MHz.
2. AC coupled, higher frequency range interstage filter design.
This design replaces the R_t resistors in Figure 34 with large
valued inductors and implements the filter just using shunt
resistors at the end of the RLC filter (here, that is just the ADC
internal differential Rin). In this case, the ADC V_CM can be tied
to the centerpoint of the bias path inductors (very much like a
Bias-T) to provide the common mode voltage and current to
the ADC inputs. These bias inductors do limit the low
frequency end of the operation where, with 1µH values,
operation from 10MHz to 200MHz is supported using the
approach of Figure 35.
3. AC coupled with output side transformer. This design includes
an output side transformer, very similar to ADC
characterization circuits. This approach allows a slightly lower
amplifier output swing (if N > 1 is used) and very easy 2nd
order low pass responses to be implemented. It also provides
the I_CM and V_CM bias to the ADC through the transformer
centertap. This approach would be attractive for higher ADC
input swing targets and more aggressive noise power
bandwidth control needs.
4. DC coupled with ADC V_CM and I_CM provided from the
amplifier. Here, DC to very high frequency interstage low pass
filters can be provided. Again, the R_S element must be low to
reduce the IR drop from the V_CM of the converter, which now
shows up on the output of the ISL55210, to the ADC input
pins. In this case, split supplies are required to satisfy the
amplifier output and input common mode range limits
discussed earlier.
Rf
ADC
+3.3V
ISL55210
Rs
Vcm1
1.2V
Ls
Icm
IN+
Cb
Rt
Ct
Vb
Ri n
Ci n
Rs
Cb
Ls
Rt
Ct
IN-
Rf
Icm
Rt ^R>t R^>_s^Rs
Vcm2 = 0.535
or 1V
vB –_VI_{bcm− I}×_cmRt_× =_{R t}V_{=cmV 2cm 2}
FIGURE 34. AC COUPLED, BROADBAND RLC INTERSTAGE FILTER DE
SIGN
RF
ADC
+3.3V
ISL55210
VCM1
1.2V
RS
Cb
1:n
ICM
Rt
Ct
IN+
Cin
Rin
RS
Cb
RF
Rt ≤30Ω
Ct
Rt
ICM
2ICM
IN-
VCM2 = 0.535
or 1V
FIGURE 36. AC COUPLED WITH OUTPUT SIDE TRANSFORMER
RF
ADC
+3.3V
ISL55210
VCM1
1.2V
RS
Cb
LS
ICM
IN+
LP
Ct
RT
Cin
Rin
RS
Cb
LS
LP
Ct
IN-
RF
ICM
VCM2 = 0.535
or 1V
Lp>>Ls
FIGURE 35. AC COUPLED, HIGHER FREQUENCY RLC FILTER DESIGN
Rf
+3.0V
ISL55210
Vcm
-1.1V
Rf
Ls
Rs
Rs
Ls
Rs ≤ 30Ω
ADC
Icm
IN+
Ct
Rt
Ct
Icm
Cin
Rin
IN-
Vcm = 0.535V
or 1V
FIGURE 37. DC COUPLED WITH A COMMON V_CM VOLTAGE FROM THE
ADC
15
FN7811.2
June 6, 2013