ISL55210

10

FN7811.0

March 2, 2011

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

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

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

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.

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

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

common mode voltage. It is useful, for this reason, to leave any

input transformer secondary centertap unconnected. The

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.

FIGURE 27. SUPPLY CURRENT vs SUPPLY VOLTAGE

1

2

3

4

5

6

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)

TEST CIRCUIT #1

30

31

32

33

34

35

36

37

38

39

40

41

42

43

44

45

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)

TEST CIRCUIT #1

50

1:1.4

1µF

1µF

85

ISL55210

+3.3V

+

-

50

50

35

35

ADT2-1T

0.1uF

200

200

200

33mA 110mW

1:1

ADT1-1WT

50

1µF

10k

PD

200

LOAD

85

1µF

FIGURE 28. TEST CIRCUIT #1