The dc servo may be enabled by checking the
DC Servo
checkbox in the GUI. To disable this function, leave the
DC Servo
checkbox in the GUI unchecked.
Two external capacitors are required for the dc servo function, with one capacitor connected between CS11
(pin 21) and CS12 (pin 20), and the second capacitor connected between CS21 (pin 19) and CS22 (pin 18).
Capacitor values up to 4.7 µF may be used. However, larger-value capacitors result in longer settling times for
the dc servo loop. A value of 1 µF is recommended for use in most microphone preamplifier applications.
The PGA2505 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 is through the PGA2505 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 is 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 is 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, VCOMIN (pin 22), allows the PGA2505 output and input to be dc
biased to a common-mode voltage between 0 V and 2.5 V. The VCOMIN pin allows for a dc-coupled interface
between the PGA2505 preamplifier output and the inputs of common, single-supply, audio analog-to-digital
converters (ADCs).
The PGA2505 may be forced to unity. Check the
Unity Gain
checkbox in the GUI to activate this function.
The zero-crossing control input is provided for enabling and disabling the internal zero-crossing detector
function. Forcing the zero-crossing input high enables the function, check the
Zero Crossing Detect
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. Zero-crossing detection can add some delay when performing gain
changes (up to 16 ms maximum for a detector timeout event). Therefore, there may be cases where the function
must be discabled. Forcing the zero cross detect 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 6. 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 GUI using the
Overload Indicator (RMS)
drop-down menu. When
the
Overload Indicator (RMS)
option is set to
5.1V
RMS differential, that is approximately −1 dB less than the
specified output voltage range. When the
Overload Indicator (RMS)
option is set to
4.0V
RMS differential, that is
approximately −3 dB less than the specified output voltage range.
The PGA2505 includes four general-purpose, programmable digital outputs, GPO1 through GPO4 (pins 2
through 5, respectively), that are controlled using the GUI 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
SBOU259 – OCTOBER 2021
PGA2505EVMV2 Evaluation Module
5
Copyright © 2021 Texas Instruments Incorporated
