Economy audio CODEC for MiniDisc (MD) home stereo and portable applications
Analog-to-Digital Converter (ADC)
The UDA1341TS accommodates slave mode only, this means that in all applications the system devices must provide the system clock. The system frequency is selectable. The options are 256fs, 384fs or 512fs. The system clock must be locked in frequency to the digital interface signals.
The UDA1341TS is partially pin compatible with the UDA1340M and UDA1344TS, making an upgrade of a printed-circuit board from UDA1340M to UDA1341TS easier. The pins that are compatible with the UDA1340M are marked in Fig.3.
Analog front end
The analog front end of the UDA1341TS consists of two stereo ADCs with a Programmable Gain Amplifier (PGA) in channel 2. The PGA is intended to pre-amplify a microphone signal applied to the input channel 2.
Input channel 1 has a selectable 0 or 6 dB gain stage, to be controlled via the L3-interface. In this way, input signals of 1 V (RMS value) or 2 V (RMS value) e.g. from a CD source can be supported using an external resistor of 12 kΩ in series with the input channel 1. The application modes are given in Table 1.
Application modes using input gain stage
The stereo ADC of the UDA1341TS consists of two 3rd-order Sigma-Delta modulators. They have a modified Ritchie-coder architecture in a differential switched capacitor implementation. The over-sampling ratio is 128.
Digital Automatic Gain Control (AGC)
Input channel 2 has a digital AGC to compress the dynamic range when a microphone signal is applied to input channel 2. The digital AGC can be switched on and off via the L3-interface. In the on state the AGC compresses the dynamic range of the input signal of input channel 2. Via the L3-interface the user can set the parameters of the AGC: attack time, decay time and output level. When the AGC is set off via the L3-interface, the gain of input channel 2 can be set manually. In this case the gain of the PGA and digital AGC are combined. The range of the gain of the input channel 2 is from −3 to +60.5 dB in steps of 0.5 dB.
AGC status detection
The AGCSTAT signal from the digital AGC is HIGH when the gain level of the AGC is below 8 dB. This signal can be used to give the PGA a new gain setting via the L3-interface and to power e.g. a LED.
The two stereo ADCs (including the AGC) can be used in four modes:
ADC1 only mode (for line input); input channel 2 is off
RESISTO (12 kΩ)
MAXIMUM INPUT VOLTAGE
• ADC2 only mode, including PGA and digital AGC (for microphone input); input channel 1 is off
• ADC1 + ADC2 mixer mode, including PGA and AGC
2 V (RMS value) input signal; note 1
• ADC1 and ADC2 double differential mode (improved ADC performance).
Present Absent Absent
6 dB 0 dB 6 dB
1 V (RMS value) input signal 1 V (RMS value) input signal 0.5 V (RMS value) input signal
Important: In order to prevent crosstalk between the line inputs no signal should be applied to the microphone input in the double differential mode.
1. If there is no need for 2 V (RMS value) input signal support, the external resistor should not be used.
In all modes (except the double differential mode) a reference voltage is always present at the input of the ADC. However, in the double differential mode there is no reference voltage present at the microphone input.
Programmable Gain Amplifier (PGA)
The PGA can be set via the L3-interface at the gain settings: −3, 0, 3, 9, 15, 21 or 27 dB.
In the mixer mode, the output signals of both ADCs in channel 1 and channel 2 (after the digital AGC) can be mixed with coefficients that can be set via the L3-interface. The range of the mixer coefficients is from 0 to −∞ dB in 1.5 dB steps.
2002 May 16