SBOU259 October 2021 PGA2505
The PGA2505 is a digitally-controlled, microphone preamplifier integrated circuit designed for amplifying the output of dynamic and condenser microphones and driving high-performance audio analog-to-digital converters (ADCs). Figure 1-1 shows a functional block diagram of the PGA2505.
The analog input to the preamplifier is provided differentially at the VIN+ and VIN− inputs (pins 24 and 23, respectively). The programmable gain amplifier can be programmed to either pass through the signal at unity gain, or apply 9 dB to 60 dB of gain to the input signal. The gain of the amplifier is adjustable over the full 9-dB to 60-dB range in 3-dB steps. The differential output of the PGA2505 is made available at VOUT+ and VOUT− (pins 15 and 14, respectively). Gain is controlled using a MSP430F5529 microcontroller and PGA2505EVMV2 graphical user interface (GUI), which can be accessed here. The microcontroller and GUI are used to program the PGA2505 gain and support functions, as shown in Figure 1-2.
A 16-bit control word is utilized to program these functions; see Figure 1-3.
The differential analog output of the PGA2505 is constantly monitored by a dc servo amplifier loop. The purpose of the servo loop is to minimize the dc offset voltage present at the analog outputs by feeding back an error signal to the input stage of the programmable gain amplifier. The error signal is then used to correct the offset. 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 Figure 1-3, or by checking the 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.