ISL55210_14 Datasheet, PDF (10/18 Pages) Intersil Corporation – Wideband, Low-Power, Ultra-High Dynamic Range Differential Amplifier

ISL55210
Typical Performance Curves V_S+ = 3.3V, T_A ≈ +25°C, unless otherwise noted. (Continued)
6 _{TEST CIRCUIT #1}
5
4
MAXIMUM DIFFERENTIAL V_P-P
OUTPUT USING DEFAULT V_CM
3
2
INTERNALLY SET V_CM
1
3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5
SUPPLY VOLTAGE (V)
FIGURE 26. DEFAULT V_CM AND MAX V_OPP vs SUPPLY VOLTAGE
45 _{TEST CIRCUIT #1}
44
43
T_A = +85°C
42
41
40
39
38
37
T_A = +25°C
36
35
T_A = -40°C
34
33
32
31
30
3.0 3.1 3.2 3.3 3.4 3.5 3.6 3.7 3.8 3.9 4.0 4.1 4.2 4.3 4.4 4.5
SINGLE SUPPLY VOLTAGE (V)
FIGURE 27. SUPPLY CURRENT vs SUPPLY VOLTAGE
Applications
Basic Operation
The ISL55210 is a very wideband, voltage feedback based,
differential amplifier including an output common mode control
loop and optional power shutdown feature. Intended for very low
distortion differential signal driving, this non-unity gain stable
device also delivers extremely low input noise terms of
0.85nV/√Hz and 5pA/√Hz. Most applications are intended for AC
coupled I/O using a single 3.3V supply. It will operate over a
single supply range of 3.0V to 4.2V. Where DC coupled operation
is desired, using split power supplies will allow the ISL55210 I/O
common mode range limits to be observed while giving either a
differential I/O or single to differential configuration.
Most applications behave as a differential inverting op amp design.
There is, therefore, an input gain resistor on each side of the inputs
that must be driven. To retain overall low output noise, these
resistors are normally of low value. The device can be powered down
to <400µA supply current using the optional disable pin. To operate
normally, this pin should be asserted high using a simple logic gate
to +V_S or tied high through a 10kΩ resistor to +V_S. When disabled,
the power dissipation drops to <1mW but, due to the inverting op
amp type architecture, the input signal will feed forward through the
external resistors giving limited isolation.
Application and Characterization Circuits
The circuit of Figure 28 forms a starting point for many of the
characterization curves for the ISL55210. Since most lab sources
and measurement devices are single-ended, this circuit converts
to differential at the input through a wideband transformer and
would also be a typical application circuit coming from a single
ended source. Assuming the source is a 50Ω impedance, the R_G
resistors are set to provide both the input termination and the
gain. Since the inverting summing nodes act as virtual ground
points for AC signal analysis, the total termination impedance
across the input transformer
this equal to n²*R_S will give
secondary
a matched
will be 2 * R_G. Setting
input impedance inside
the bandwidth of the transformer (where "n" is the turns ratio).
The amplifier gain is then set by adjusting the feedback resistors
values. Since the ISL55210 is a VFA design, increasing the
feedback resistor to get higher gain does not directly reduce the
bandwidth as it would with a CFA based design. This gives
increased flexibility in the input turns ratio and overall gain
setting (while holding a matched input impedance) over
alternate solutions.
50
1:1.4
VI _1µF
ADT2-1T
+3.3V
33mA 110mW
200
RF
10k
50
+
RG
200
PD
0.1uF
VCM
ISL55210
RG
-
50
85
1µF
35
200
VO LOAD
35
1µF ⁸⁵
50
1:1
1µF ^VM
ADT1-1WT
RF
200
FIGURE 28. TEST CIRCUIT #1
Working with a transformer coupled input as shown in Figure 28,
or with two DC blocking caps from a differential source, means
the output common mode voltage set by either the default
internal V_CM setting, or a voltage applied to the V_CM control pin,
will also appear as the input common mode voltage. This
provides a very easy way to control the ISL55210 I/O common
mode operating voltages for an AC coupled signal path. The
internal common mode loop holds the output pins to V_CM and,
since there is no DC path for an I_CM current back towards the
input in Figure 28, that V_CM setting will also appear as the input
common mode voltage. It is useful, for this reason, to leave any
input transformer secondary centertap unconnected. The
internally set V_CM voltage is referenced from the negative supply
pin. With a single 3.3V supply, it is very close to 1.2V but will
change with total supply voltage across the device as shown in
Figure 26.
10
FN7811.2
June 6, 2013