New products coming on the market with single-supply devices and a built-in charge pump to generate a negative supply rail are rapidly changing the landscape of audio and video products.
The first people to come up with interface standards were in the broadcast market. Attaching multiple products together to create a final product required a standard to make sure the output of one product wouldn't overdrive the next, and that the ideal amount of gain or attenuation could be added to a signal with ease.
Traditional interface standards were given at +4 dBu and -10 dBV (that's 1.23 Vrms and 0.316 Vrms for those without a calculator or the will to make the calculations). For many years, this was what differentiated consumer and professional equipment. Take any standalone CD player from the early 1990s and you'll quickly see the standard I'm talking about.
In the late 1980s and early 1990s people got a little pickier about what they connected to their television. The days of just hooking up the VHS machine via the coaxial aerial connection was slowly coming to an end. With the advent of higher quality audio sources (NICAM stereo, Laserdisc, etc.), audio started being transported across stereo RCA jacks - just like your Hi-Fi did.
SCART standard
Around the same time, SCART connectors in Europe started to become popular, too (Figure 1). SCART gave manufacturers a whole new playground of standards to play with. Not only were the connectors new, but the levels that people could play increased significantly.
Figure 1 - SCART Connector.
As with any interconnect, the higher the signal you push through, the higher the dynamic range you can get in the receiver as receivers typically have a fixed amount of noise. Along with the new connector came standards for voltage swing and for the expected impedance a receiver should impose on the driver. The SCART standard eventually grew to be 2 Vrms (5.6 Vpp) with an input impedance of 10 kΩ.
A ground-biased 5.6-Vpp signal is tougher to generate than one may at first think. Consider for a moment the products we connect in a home entertainment environment: DVD players, set-top boxes, gaming consoles, AV receivers, etc. The output of a typical 5-V DAC is around 4 Vpp, biased around 2.5 V. Lower operating voltage DACs will have even less signal swing biased around a lower voltage. Getting this signal into the outside world requires some level of gain and a level of buffering.
This nicely brings us to output topologies for taking DAC outputs to the real world. There are three main methods of doing this:
bipolar-supply operational amplifier (op amp) with a split rail power supply;
single-supply op amp with a DC bias of VDD/2, and a DC blocking capacitor on the output;
single-supply op amp with a built-in charge pump to generate a negative supply rail, giving enough swing and a ground-biased output.
Each of these topologies has their own advantages and disadvantages.
Solution #1: Bipolar-supply op amp with fixed input.
Many systems use a standard op amp in an inverting or non-inverting format to take the unbalanced output of a DAC or SoC to the outside world. This, by far, is the simplest way to create a 5.6-Vpp swing output, biased around GND.
Figure 3 - Ideal ground biased output from a dual supply output stage.
Using this method, the designer can fine tune the sound they are looking for, cherry pick the op amp they want, and even insert some filtering in the output path.
However, many consumer audio systems aren't running voltage supplies higher than +5 V, and the hope of having a negative supply rail adds to COB. Some systems on the market generate the rails for audio line-level amplifiers, however, normally at the cost of two additional low dropout regulators (LDOs), as well as the passives needed to set up the device.
Conclusions:
Benefits: Nice to have, if you can afford it in your system.
Concerns: Cost, PCB space, form factor.
Solution #2: Single-supply op amp with DC bias of VDD/2 and a DC-blocking cap.
Figure 4 - Single-supply line driver op amp.
Here's another old favorite. In the past, a low-cost method used in many systems was to run a single-supply op amp from a high-voltage rail of 8 V+. However, many systems don't like receiving a signal with a DC bias on it - designers normally add DC-blocking capacitors to ensure only AC signals leave their product.
Figure 5 - The output from a traditional single supply opamp.
In such a system, the capacitor charges up with DC as the receiver completes a path to a lower potential. During normal operation this wouldn't be much of a problem. However, as soon as the amplifier changes state (i.e., is switched on, switched off, or moves to low-power mode), that DC is going to discharge, causing audible pops and clicks.
Additionally, those who have played with analog filter designs will see what happens when you put a capacitor directly in the audio chain. The nature of a capacitor's impedance is that at low frequencies, it displays a higher resistance to the signal. In other words, they don't just block DC. DC-blocking capacitors introduce a high-pass filter effect to the output of your system. One of the few ways of moving the 3-dB point of the filter is by getting a higher value capacitor. The following equation calculates the 3-dB filter point for a RC filter:
f3db = 1 / 2πRlCo
Where Rl is the load of the next device in the chain and Co is the output capacitance. Large value capacitors typically cost more and take more PCB space.
Conclusions:
Advantages: Low cost.
Disadvantages: May need a dedicated VCC, potential pops and clicks, poor bass response.
Solution #3: A single-supply device with a built in charge pump to generate a negative supply rail.
Figure 6 - TI's DRV60x family, which integrates a bipolar op amp and charge pump.
This is one of the newest methods used to generate a higher-voltage signal swing output and maintaining a ground-biased output.
DirectPath technology patented by TI integrates a charge pump into the device, creating an internal negative supply. This allows the amplifier in the device to run in a bipolar (dual supply) mode, creating a potential 6-V+ swing with a ground-biased output.
Figure 7 - DirectPath gives bipolar performance with single-supply ease of integration.
Such technology offers the best of both worlds. The performance and the advantages of a ground-biased output offered by a bipolar power supply gives pop-free performance, along with a direct connection to the next device in the chain - with no DC-blocking caps. Bass response is also a lot more natural. Furthermore, running from a +3.3-V supply means the device can run from a typical system voltage (usually found in digital processors, etc). No need now for dedicated higher voltage rails, or, usually overlooked, the need to lay out extra power traces across your system to the output amplifier.
For the majority of home audio systems, an audio performance of 107 dB is in excess of any DACs in their systems. It's worth pointing out that some systems may need to move to a traditional dual supply output driver design. Typical systems mostly would be deemed "professional" AV receivers whose converters are running in the 110-dB+ range. A large range of op amps are available for these applications as well.
Conclusions:
Advantages: The best of both worlds - pop free, clear bass response, easy-to-lay-out device - ideal for 95% of the home audio market.
Disadvantages: Performance limitation at around 107-dB dynamic range - usually not a problem in the set-top box, portable DVD player and gaming console markets.
Many designers are finally beginning to ask themselves why they need higher voltage rails in their systems, if all they are doing is driving the output line drivers. New products coming on the market with single-supply devices and a built-in charge pump to generate a negative supply rail, such as with DirectPath, are rapidly changing the landscape of audio and video products.
About the Author: Dafydd Roche is the Home Entertainment and Professional Audio Marketing Manager for TI's High Performance Analog group. A graduate from the University of York (UK), Dafydd pours his passion and knowledge of audio and music making into his work, doing his part to help enable audio design engineers to make products that end users can't wait to use. In between a hectic life of customer visits, internal meetings and tradeshows, Dafydd still manages to find time to make and record music with fellow musicians in the Dallas area.
