Texas Instruments PGA2505 Evaluation Module PGA2505EVM PGA2505EVM 데이터 시트
제품 코드
PGA2505EVM
SCLK
DC
CM
ZC
OR
D4
D3
D2
D1
0
0
G5
G4
G3
G2
G1
G0
Data Ignored
Data Ignored
DC
CM
ZC
OR
D4
D3
D2
D1
0
0
G5
G4
G3
G2
G1
G0
High Impedance
High Impedance
CS
SDI
SDO
DC Servo Enable
(Active Low)
(Active Low)
CM Servo Enable
(Active High)
(Active High)
Over-Range Indicator Bit
(0 = 5.1V
, 1 = 4.0V
)
RMS
RMS
Data for GPO4
Data for GPO2
Data for GPO1
Preamplifier Gain
where N = G[5:0]
where N = G[5:0]
DEC
For N = 0
Gain = 0dB
Gain = 0dB
For N = 1 to 17
Gain (dB) = 6 + 3N
Gain (dB) = 6 + 3N
For N = 18 to 31
Gain = 60dB
Gain = 60dB
Data for GPO3
Zero Crossing Detect
(Active High)
(Active High)
Introduction
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The four-wire serial port interface is used to program the PGA2505 gain and support functions. A 16-bit
control word is used to program these functions, as
control word is used to program these functions, as
illustrates. A serial data output pin provides
support for daisy-chaining multiple PGA2505 devices on a single serial interface bus.
Figure 2. PGA2505 Serial Port Protocol
The differential analog output of the PGA2505 is constantly monitored by a dc servo amplifier loop. When
enabled, 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 setting the DC bit in the serial control
word to '1', as shown in
enabled, 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 setting the DC bit in the serial control
word to '1', as shown in
.
Two external capacitors are required for the dc servo loop to function properly, with one capacitor
connected between C
connected between C
S11
and C
S12
(pins 21 and 20), and the second capacitor connected between C
S21
and C
S22
(pins 19 and 18). Capacitor values up to 4.7
µ
F are recommended. However, larger valued
capacitors can be used, but will result in longer settling times for the dc servo loop. A value of 1
µ
F is
sufficient for use in most microphone preamplifier applications.
The PGA2505 also includes a common-mode servo function. This function can be enabled and disabled
using the CM bit in the serial control word. When enabled, the CM servo provides common-mode negative
feedback at the input differential pair, resulting in very low common-mode input impedance. Note that 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, such as with a decoupling capacitor. In this case,
the only connection between the source and the ground will be through the PGA2505 preamplifier input
resistance.
using the CM bit in the serial control word. When enabled, the CM servo provides common-mode negative
feedback at the input differential pair, resulting in very low common-mode input impedance. Note that 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, such as with a decoupling capacitor. In this case,
the only connection between the source and the ground will be through the PGA2505 preamplifier input
resistance.
With CM enabled, 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 without increasing the input common-mode
current. Increasing common-mode current degrades common-mode rejection. Using the common-mode
servo loop, overall common-mode rejection (or CMRR) can be improved by suppressing low and medium
frequency common-mode interference.
source, not by input impedance of the amplifier. Therefore, input common-mode interference can be
reduced by lowering the common-mode input impedance without increasing the input common-mode
current. Increasing common-mode current degrades common-mode rejection. Using the common-mode
servo loop, overall common-mode rejection (or CMRR) 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 10nF. Beyond this limit, stable servo operation is
not ensured.
(including the microphone cable capacitance) of up to 10nF. Beyond this limit, stable servo operation is
not ensured.
The common-mode voltage control input, named V
COM
IN (pin 22), allows the PGA2505 output and input to
be dc-biased to a common-mode voltage between 0V and +2.5V. This architecture allows for a dc-coupled
interface between the PGA2505 preamplifier output and the inputs of common single-supply audio
analog-to-digital (A/D) converters.
interface between the PGA2505 preamplifier output and the inputs of common single-supply audio
analog-to-digital (A/D) converters.
4
PGA2505EVM
SBOU078 – June 2009