16, 28 23, 7
Supply Voltage: To minimize noise, the analog and digital circuits
in the ISD2500 series devices use separate power busses. These
voltage busses are brought out to separate pins and should be tied
together as close to the supply as possible. In addition, these
supplies should be decoupled as close to the package as possible.
Microphone: The microphone pin transfers input signal to the on-
chip preamplifier. A built-in Automatic Gain Control (AGC) circuit
controls the gain of this preamplifier from –15 to 24dB. An external
microphone should be AC coupled to this pin via a series capacitor.
The capacitor value, together with the internal 10 KΩ resistance on
this pin, determines the low-frequency cutoff for the ISD2500 series
passband. See Winbond’s Application Information for additional
information on low-frequency cutoff calculation.
Microphone Reference: The MIC REF input is the inverting input
to the microphone preamplifier. This provides a noise-canceling or
common-mode rejection input to the device when connected to a
Automatic Gain Control: The AGC dynamically adjusts the gain of
the preamplifier to compensate for the wide range of microphone
input levels. The AGC allows the full range of whispers to loud
sounds to be recorded with minimal distortion. The “attack” time is
determined by the time constant of a 5 KΩ internal resistance and
an external capacitor (C2 on the schematic of Figure 5 in section
11) connected from the AGC pin to VSSA analog ground. The
“release” time is determined by the time constant of an external
resistor (R2) and an external capacitor (C2) connected in parallel
between the AGC pin and VSSA analog ground. Nominal values of
470 KΩ and 4.7 µF give satisfactory results in most cases.
Analog Input: The analog input transfers analog signal to the chip
for recording. For microphone inputs, the ANA OUT pin should be
connected via an external capacitor to the ANA IN pin. This
capacitor value, together with the 3.0 KΩ input impedance of ANA
IN, is selected to give additional cutoff at the low-frequency end of
the voice passband. If the desired input is derived from a source
other than a microphone, the signal can be fed, capacitively
coupled, into the ANA IN pin directly.
Publication Release Date: June 2003