7 (13)

DA6180B.003

17 September, 2014

TYPICAL APPLICATION (Continued)

Note 2: AGC Capacitor

The AGC and DEC capacitors must have low leakage currents due to very small signal currents through the

capacitors. The insulation resistance of these capacitors should be at minimum 100 MΩ. Also probes with at

least few 100 MΩ impedance should be used for voltage probing of the AGC and DEC pins. Electrolytic AGC

capacitor should have voltage rating at least 25 V for low enough leakage. DEC capacitor can be low leakage

chip capacitor.

It is recommended to connect both AGC and DEC capacitors to VDD (see application figures 1, 2 and 3)

although VSS connection is also possible. The VDD connection provides better supply noise immunity because

signals are referenced to VDD. Additionally leakage currents are minimized in this connection because in power

down the AGC pin voltage is pulled to VDD (to minimum AGC gain) then corresponding to zero voltage over the

AGC capacitor.

Note 3: Power Down / Fast Startup Control

Both power down and fast startup are controlled using the PDN pin. The device is in power down (turned off) if

PDN = VDD and in power up (turned on) if PDN = VSS. Fast startup is triggered automatically by the falling edge

of PDN signal, i.e., controlling device from power down to power up. The VDD must be high before falling edge of

PDN to guarantee proper operation of fast startup circuitry. Before power up the device should have been kept in

power down state at least 50ms. This guarantees that the AGC capacitor voltage has been completely pulled to

VDD during power down. The startup time without proper fast startup control can be over minute but with fast

startup it is shortened typically to a few seconds.

Note 4: Optional Control for AGC On/Hold

AON control pin has internal pull up which turns AGC circuit on all the time if AON pin is left unconnected.

Optionally AON control can be used to hold and release AGC circuit. Stepper motor drive of analog clock or

watch can produce disturbing amount of noise which can shift the input amplifier gain to unoptimal level. This can

be avoided by controlling AGC hold (AON=VSS) during stepper motor drive periods and releasing AGC

(AON=VDD) when motors are not driven. The AGC should be in hold only during disturbances and kept on other

time released since due to leakage the AGC voltage can change slowly even when in hold.

Note 5: Ferrite Antenna

The ferrite antenna converts the transmitted radio wave into a voltage signal. It has an important role in

determining receiver performance. Recommended antenna impedance at resonance is around 100 kΩ.

Low antenna impedance corresponds to low noise but often also to small signal amplitude. On the other hand

high antenna impedance corresponds to high noise but also large signal. The optimum performance where

signal-to-noise ratio is at maximum is achieved in between.

The antenna should have also some selectivity for rejecting near signal band disturbances. This is determined by

the antenna quality factor which should be approximately 100. Much higher quality factor antennas suffer from

extensive tuning accuracy requirements and possible tuning drifts by the temperature.

C

B

C

f

Q

Q

L

f

R

res

ant

ant

res

ant

⋅

⋅

=

⋅

⋅

=

⋅

⋅

⋅

=

π

π

π

2

1

2

2

Equation 1.

B

f

Q

res

Equation 2.

Table 4 on next page presents some antenna suppliers for time signal application.