settling times for the DC servo loop. A value of 1 µF is recommended for use in most microphone preamplifier
applications.
The PGA2500 includes a common-mode servo function. This function is enabled and disabled using the CM bit
in the serial control word, as shown in
CM Servo
checkbox in the GUI. When
enabled, the servo provides common-mode negative feedback at the input differential pair, resulting in very low
common-mode input impedance. The differential input impedance is not affected by this feedback. This function
is useful when the source is floating, or has a high common-mode output impedance. In this case, the only
connection between the source and the ground will be through the PGA2500 preamplifier input resistance.
In this case, input common-mode parasitic current is determined by high output impedance of the source,
not by input impedance of the amplifier. Therefore, input common-mode interference can be reduced by
lowering the common-mode input impedance while not increasing the input common-mode current. Increasing
common-mode current degrades common-mode rejection. Using the common-mode servo, overall common-
mode rejection can be improved by suppressing low and medium frequency common-mode interference.
The common-mode servo function is designed to operate with a total common mode input capacitance (including
the microphone cable capacitance) of up to 10 nF. Beyond this limit, stable servo operation is not ensured.
The common-mode voltage control input, named VCOMIN (pin 25), allows the PGA2500 output and input to be
DC biased to a common-mode voltage between 0 V and +2.5 V. This allows for a DC-coupled interface between
the PGA2500 preamplifier output and the inputs of common single-supply audio A/D converters.
A dedicated 0-dB input (pin 8) is provided so that the gain of the PGA2500 may be forced to unity. Check the
Unity Gain
checkbox in the GUI to activate this function.
The zero-crossing control input, named ZCEN (pin 9), is provided for enabling and disabling the internal zero-
crossing detector function. Forcing the ZCEN input high enables the function, check the
ZCEN
checkbox in the
GUI to activate this function. Zero-crossing detection is used to force gain changes on zero crossings of the
analog input signal. This limits the glitch energy associated with the switched gain network, thereby minimizing
audible artifacts at the preamplifier output. Since zero-crossing detection can add some delay when performing
gain changes (up to 16 ms maximum for a detector timeout event), there may be cases where the user may wish
to disable the function. Forcing the ZCEN input low (unchecking the checkbox in the GUI) disables zero-crossing
detection, with gain changes occurring immediately when programmed.
An overflow indicator output, OVR, is provided at pin 5. The OVR pin is an active high, CMOS-logic-level output.
The overflow output is forced high when the preamplifier output voltage exceeds one of two preset thresholds.
The threshold is programmed through the graphical user interface using the
Overload Indicator (RMS)
drop-
down menu. When this Overload Indicator option is set to "5.1V" RMS differential, that is approximately −1 dB
below the specified output voltage range. When the Overload Indicator option is set to "4.0V" RMS differential,
that is approximately −3 dB below the specified output voltage range.
The PGA2500 includes four general-purpose programmable digital outputs, named GPO1 through GPO4 (pins
1 through 4, respectively), which are controlled via the GUI as checkboxes. All four pins are CMOS-logic-level
outputs. These pins may be used to control relay drivers or switches used for external preamplifier functions,
including input pads, filtering, polarity reversal, or phantom power.
Product Overview
4
PGA2500EVMV2 Evaluation Module
SBOU257 – JUNE 2021
Copyright © 2021 Texas Instruments Incorporated
