LT5537 datasheet(13/16 Pages) LINER

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LT5537

5537fa

APPLICATIO S I FOR ATIO

INPUT POWER (dBm)

–100

0.5

1.0

1.5

2.0

2.5

–80

–60

–40

–20

5537 F17

020

TA = 25°C

VCC = ENBL = 3V

Figure 11. 10kHz Operation

Design 2 is the application circuit (Figure 13) used for

characterization in this data sheet. This is a robust general

purpose design which can operate as low as 1.3MHz.

Optional filter capacitor (C6 = 33nF) together with the on-

chip capacitor set the dominant pole at 740Hz. The input

pole associated with the AC coupling capacitors (C1, C2 =

100pF) is at 1.3MHz which is beyond the loop cut-off

frequency of 160kHz. The zero is at an even higher

frequency and can be safely ignored. This design has a

stability phase margin of 84 degrees, resulting in a very

well damped response to any input biasing transients.

Design 3 features fast settling. This design is appropriate

when fast response in the presence of input biasing

transients is required, and very low frequency operation is

not needed.

Design 4 demonstrates the possibility of operating the

LT5537 at very low frequency (<10kHz) by configuring the

offset cancellation loop for very low bandwidth. The re-

sponse of this circuit at 10kHz is plotted in Figure 11.

Offset Cancellation Loop and the Timing Response

The input of the LT5537 is AC coupled, and the on-chip DC

biasing is automatically regulated as described above. But

if the DC component of the input signal has any transient

step with sufficiently short rise or fall time (for example the

output of an active RF switch has a biasing shift between

switching states), a transient voltage pulse is induced by

the displacement current needed to charge the input AC

coupling capacitor. Also, if the pulse frequency or the

repetition rate is within the loop bandwidth of the offset

cancellation circuit, the LT5537 will respond to the in-

duced voltage pulse in the same way it nulls out its internal

DC offset, even though the chip is DC isolated from the

input signal.

If the external capacitor (C6) is used to extend the low

frequency response of the LT5537, then this will also

lengthen the response time of the DC offset cancellation

circuit. In the presence of DC steps or glitches at the input,

the transient response of the slowed offset cancellation

loop will be superimposed on the faster logarithmic detec-

tor output, degrading the overall response time of the chip.

The sensitivity of the LT5537 is very high. An input biasing

step with amplitude of 0.5mV can generate a output

voltage response of 400mV before the input voltage tran-

sient dissipates or the offset cancellation loop nulls out the

transient, whichever occurs first.

One way to prevent the input signal containing a biasing

transient from degrading the timing response is to design

the offset cancellation loop to have a high bandwidth,

allowing faster settling. Design 3 in Table 3 is suitable for

this purpose, but will not operate below 20MHz.

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