TLV320AIC3254: TLV320AIC design query

Sudarshan,

The TLV320AIC3254 is a good fit for a digital stethoscope system. The Digital Stethoscope Medical Development Kit uses a TMS320C5515/C5517 EVM with a daughter card with the digital Stethoscope analog front end. With regards, to your questions:

1) The TLV320AIC3254 has three power supplies:

• AVDD (analog). This supply has a range of 1.5V - 1.95V, with a nominal recommended supply of 1.8V. For best performance, use an LDO regulating to 1.8V from a higher supply on the system.
• IOVDD (digital I/O pins). This supply has a range of 1.1V - 3.6V. Share the same IOVDD supply as the I/O of other devices interfacing to the TLV320AIC3254 codec. For example, the same I/O supply as the TMS320C55xx device.
• DVDD (digital core logic). This supply has a range of 1.26 - 1.95V, with a nominal recommended supply of 1.8V. You  can use a DC-DC converter or LDO to regulate 1.8V. For best performance, do not use the same supply for AVDD as DVDD since the high frequency switching of digital logic will add noise to the DVDD power supply.  Thus keep the analog power and grounds separate from the digital power and ground.

For typical battery operated systems, the power management takes power form battery and regulates it to the required supplies. If using rechargeable batteries, or USB power start with a battery charger IC. Selection of the charger depends on the number of cells, battery chemistry, and charge current. For example, for a single cell Li-Ion with support to 2A charge, a BQ24250 might be a good fit. Then follow this with regulation for I/O, digital logic, and analog. For example, a 3.3V regulator for I/O and digital supply could be a TPS63036 or a TPS767D3xx. 1.8V analog regulation could be a REG1117. Selection of these regulators depend on the input voltage, output voltage, and total peak power consumed by the devices connected to this supply.

Refer to MDK or EVM schematics for examples on power management circuits and layout.

2. SPI and I2C are used by the TLV320AIC3254 for control. I2S is used for data transfers to the TLV320AIC3254. Selection of SPI and I2C is highly dependent on the other devices on the board, how fast the microprocessor/DSP can support the transfers, number of routing traces supported on the board, the peripherals are available on the microprocessor/DSP, and the clock speed supported on these interfaces.

• I2C is a simple two-wire interface that allows for several devices to be wired to the same bus. The TLV320AIC3254 supports 100 and 400kHz transfers. Each transfer has overhead of a device address and acknowledgement bits.
• SPI is a four wire interface. Several devices can share the same bus, if a specific chip select line is wired to each device and only one transfer at a time is allowed by the software. Several devices can also have individual bus. The TLV320AIC3254SPI supports SPI clock rates of 10 MHz at 1.8V, so this interface is much faster than I2C and does not have the extra transmission of address and acknowledge as in I2C.

In summary, SPI offers faster transfer rates at the expense of extra trace routing on the board and requires specific chip selects or another SPI bus for each device the microprocessor/DSP needs to interface. For most control type data, the extra speed is not needed since the registers are set at boot up. The convenience of routing a couple lines tends to prefer I2C or SPI.

I2S is always needed since this interface transfers the data from the ADC and DAC.

3) I2C pull-ups are selected depending on the total bus capacitance to match the maximum rise time of the I2C signal. The bus capacitance and and sink current of the I2C I/O determine the minimum rise time of the I2C signal:

At 1.8V:

Rmin = (Vdd - Vol)/I = (1.8 - 0.4)/ 4mA = 350

4) The MDK has specific analog front end components tailored for a digital stethoscope to connect to a C55xx EVM:

• MicBias routing to power the microphones.
• Headphone jack for microphone in single ended mode.
• A pre-amplifier (PGA) to boost the low signal from the microphones to the AIC3254. This allows the input signal to match the dynamic range of the AIC3254 so as to maintain high precision during conversion. Otherwise, the low signal will only cover a couple bits of the conversion, not the full range of conversion.
• Low pass filters to block high frequency components not desired from a stethoscope signal that just add noise to the conversion.
• Software/ demo application showing how to interface a C55xx DSP and AIC3254 to record heart in bell or diaphragm mode, detect heart beat, and display on screen.

The TLV320AIC3254 EVM has on on-board microphones or a headphone jack connected to the AIC3254 in differential mode. This could be used for a digital stethoscope, but might need to use different microphones, add pre-amplifier, and low pass filter. This EVM is meant to use USB to send the data to a PC and not to interface directly to a C55x EVM.

best regards,

  Pedro