ISL55211 Datasheet, PDF (16/20 Pages) Intersil Corporation – Wideband, Low Noise, Low Distortion, Fixed Gain, Differential Amplifier

ISL55211
+3.3V
ISL55211
VCM1
1.2V
LS
RS
CB
VB
RS
CB
LS
ICM
RT ^CT
RT _CT
ICM
R_t >R_s
V_b −I_cm×R_t =V_cm2
ADC
IN+
CIN
RIN
IN-
VCM2
FIGURE 36. AC- COUPLED BROADBAND RLC INTERSTAGE FILTER
DESIGN
2. AC-coupled, higher frequency range interstage filter design.
This design replaces the R_T resistors in Figure 35 with large
valued inductors and implements the filter just using shunt
resistors at the end of the RLC filter. 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 37.
ADC
+3.3V
LS
ICM
IN+
ISL55211
VCM1
1.2V
RS
CB
RS
CB
RT
LS
Lp >>Ls
LP
CT
LP
CT
ICM
CIN
RIN
IN-
V_CM2
FIGURE 37. AC-COUPLED, HIGHER FREQUENCY RLC INTERSTAGE
FILTER DESIGN
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 or
3rd 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. Figure 1 on page 1 is an
example showing this approach.
ADC
+3.3V
ISL55211
RS
CB _1:N
VCM1
1.2V
RS ^CB
ICM
RT
CT
IN+
CIN
RIN
CT
RT
IN-
RT < 30
ICM
2ICM
VCM2
FIGURE 38. AC-COUPLED WITH OUTPUT SIDE TRANSFORMER
4. DC-coupled with ADC V_CM and I_CM provided from the
amplifier. Here, DC to very high frequency interstage low pass
filter 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 ISL55211, to the ADC input
pins.
ADC
+3.3V
ISL55211
VCM
LS
RS
RS
LS
Rs ≤30Ω
ICM
IN+
CT
RT
CIN
RIN
CT
ICM
IN-
VCM2
FIGURE 39. DC-COUPLED WITH A V_CM VOLTAGE FROM THE ADC
16
FN7868.0
June 21, 2011