Datasheet

PGA2500 Product Overview
1-3
Introduction
Figure 1−2. PGA2500 Serial Port Protocol
Data Ignored DC CM 0 OL D4 D3 D2 D1 0 0 G5 G4 G3 G2 G1 G0 Data Ignored
CS
SCLK
SDI
High Impedance
DC Servo Enable
(active low)
CM Servo Enable
(active high)
Overload Indicator Bit
(0 = 5.1Vrms, 1 = 4.0Vrms)
Data for GPO4
Data for GPO3
Data for GPO2
Data for GPO1
DCCM 0 OLD4D3D2D1 0 0 G5G4G3G2G1G0 HighImpedanceSDO
Preamplifier Gain
where N = G[5:0]
DEC
For N = 0
Gain = 0dB
For N = 1 to 56
Gain (dB) = 9 + N
ForN=57to63
Gain (dB) = 65
The differential analog output of the PGA2500 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 disabled by driving the DCEN
input
(pin 7) high or setting the DC
bit in the serial control word to 1. Normally, the
DCEN pin is connected to DGND and the DC bit is set to 0 to enable the DC
servo.
Two external capacitors are required for the DC servo function, with one ca-
pacitor connected between C
S11
and C
S12
(pins 24 and 23), and the second
capacitor connected between C
S21
and C
S22
(pins 22 and 21). Capacitor val-
ues up to 4.7µF may be utilized. However, larger valued capacitors will result
in longer settling times for the DC servo loop. A value of 1µF is recommended
for use in most microphone preamplifier applications.
The PGA2500 includes a common-mode servo function. This function is en-
abled and disabled using the CM bit in the serial control word, as shown in
Figure 1−2. When enabled, the servo provides common-mode negative feed-
back at the input differential pair, resulting in very low common-mode input im-
pedance. 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 out-
put impedance of the source, not by input impedance of the amplifier. There-
fore, input common-mode interference can be reduced by lowering the com-
mon-mode input impedance while not increasing the input common-mode cur-
rent. Increasing common-mode current degrades common-mode rejection.
Using the common-mode servo, overall common-mode rejection can be im-
proved by suppressing low and medium frequency common-mode interfer-
ence.