I often get questions about changing a bipolar supply circuit (using positive and negative supplies) to a circuit that uses no negative supply. Operational amplifiers do not care whether or not they have a negative supply, but do require a certain range of total supply voltages. So, for +15V / -15V performance, it would seem that we may need a positive supply voltage of +30V and ground. However, the input should be near halfway between the two supply voltages, or in the vicinity of +15V with a single +30V supply. However, using a rail-to-rail input op-amp would allow you to take the input down to ground potential.

Let’s take a look at a circuit in the Texas Instruments PCM1792ADBPCM1792ADB DAC data sheet (http://focus.ti.com/general/docs/lit/getliterature.tsp?genericPartNumber=pcm1792a&fileType=pdf). Figure 36 (Page 35) of that data sheet shows a circuit to convert the DAC differential current output to a single-ended voltage output using the NE5534 and the LT1028 op-amps with +/-15V rails.

The NE5534 will operate on as little as +/-3V supplies, which implies that it will operate on a +6V single supply (plus ground, of course). This is not quite the +5V we might like, but it is a single supply. However, how close can the input and output get to ground (the negative supply pin) in this case and the supply voltage and how good are the specifications at low supply voltages?

A look at the NE5534NE5534 data sheet shows the common-mode input voltage range minimum is +12V / -12V with 15V supplies, so must not come any closer to the supply rails than 3V. Therefore, the input voltage must be a minimum of ground plus 3V and can not be more than the supply minus 3V and, with a 6V single supply, is constrained to be +3V, period. If you have a single +7V to +15V supply, that single supply can be used and you must bias pin 3 of the NE5534 to a positive voltage. How high it needs to be is determined by the output current range of the PCM1792ADBPCM1792ADB DAC and the feedback resistor of the NE5534NE5534.

The maximum peak-to-peak output swing of the NE5534 is 24V with +/-15V supplies, meaning that the output swing is 6V less than the total supply voltage, so the output can go to 3V from either rail.

Assuming we power the NE5534 with a single +15V supply, we find that the maximum output swing is 15V – 3V – 3V = 9Vp-p. The two negative 3V quantities are the voltages the output must keep from the positive and negative supply rails. With the 7.8 mAp-p output current of the DAC and the 750 Ohm feedback resistor of the NE5534NE5534, the output voltage swing is 5.85V, well below the 9V swing we can accept. (The allowable swing for a 12V supply is just 6V and too marginal to consider.)

The range for the bias on the non-inverting input pin of the NE5534 is Minimum Vbias = Min Out – Min DAC output current * 750 Ohms = 3V – 2.3 mA * 750 Ohms = 1.275V to  Maximum Vbias = Max Out – Max DAC output current * 750 Ohms = 12V – 2.3 mA * 750 Ohms = 4.425V. With a range of 1.275V to 4.425V, using a standard reference voltage would mean using a 2.048V, or 2.5V or 4.096V reference. Since 2.85V is the mid point of usable references, we will choose a 2.5V reference as the closest to 2.85V. The LT1019 from Linear Technology is a good choice for this bias and one of them may be used for all four NE5534s in a stereo circuit. With 9Vp-p output the maximum NE5534 feedback resistor is 9V / 7.8mA = 1,153 Ohms, so the 750 Ohms in the schematic is satisfactory and we can keep the same gain.

Now we need to ensure that the output of U3, the LT1028, remains within allowable limits. The data sheet indicates that the input voltage and the output voltage can both go to within 4V of both rails. A similar analysis as used for the NE5534NE5534 will reveal that R6 should be returned to a potential of 6.9V to 8.2V. A reference voltage of 7V, as provided by the LT1021BC, will do well here.

Of course, a worst case analysis should be performed, after which we may decide to use high precision resistors and possibly decide upon different values for the references, but I think that 1% resistors and the references chosen are probably satisfactory.

The key things to remember in modifying a reference design for your own purpose is to remember that the total supply for op-amps is needed, not necessarily a negative supply, stay with all device specifications and do your due diligence in design. Oh, and do not forget to document where your original circuit originated and why you made your changes.