[Repost] How can multiplying DAC be used in applications other than DAC?

赵玉田

[Repost] How can multiplying DAC be used in applications other than DAC? [Copy link]

You may know that some DACs contain R2R networks that can generate a reference voltage at the output. These resistors are precision resistors. They are usually used to switch currents according to the digital value sent to the DAC, thereby generating a voltage at the output amplifier. When using multiplying DACs, the output amplifier is not integrated. This makes it possible to realize some unconventional applications and use the R2R network as a resistor.

Interested? Today, Thomas Tzscheetzsch, field applications engineer manager for ADI healthcare industry customers, will explain to you "How multiplying DACs can be used for applications other than DACs."

Most DACs Operating with a fixed positive reference voltage, the output voltage or current is proportional to the product of the reference voltage and the set digital code. This is not the case with so-called multiplying digital-to-analog converters (MDACs), where the reference voltage can be varied, typically within a range of ±10 V. The analog output can therefore be influenced (dynamically in both cases) by the reference voltage and the digital code.

Applications

With appropriate wiring, the module can output an amplified, attenuated or inverted signal (relative to the reference signal). Its fields of application therefore include waveform generators, programmable filters and PGAs (programmable gain amplifiers), as well as many other applications where offset or gain must be adjusted.

Figure 1. Circuit with variable gain (PGA)

Figure 1 shows an AD5453 14-bit MDAC with a downstream amplifier that amplifies or attenuates the signal depending on the programmed digital code of the DAC.

Circuit Calculations

The output voltage (VOUT) of this circuit is calculated as follows:

In addition to the gain and the set digital code D of the DAC, the output voltage is also affected or limited by the supply voltage of the operational amplifier. In the case shown, the supply voltage of the ADA4637-1 amplifier is ±15 V and should output a maximum voltage of ±12 V, so its control range is large enough. The gain is determined by resistors R2 and R3:

All resistors (R1 to R3) should have the same temperature coefficient of resistance (TCR), but they do not have to be the same as the TCR of the internal resistors of the DAC. Resistor R1 is used to adjust the DAC internal resistor (RFB) according to R2 and R3 and the following relationship:

When selecting the resistor, it is important to ensure that the op amp is still in the operating range at the maximum input voltage (the DAC can handle ±10 V at VREF). It should also be noted that the amplifier's input bias current (IBIAS) is amplified by the resistor (RFB + R2|| R3), which has a considerable impact on the offset voltage. The ADA4637-1 op amp with its ultra-low input bias current and ultra-low input offset voltage (according to the data sheet) was chosen for this reason. To prevent instability or so-called ringing in the closed-loop control system, a 4.7 pF capacitor is inserted between IOUT and RFB; this is particularly recommended for fast amplifiers. As mentioned earlier, the amplifier's offset voltage is amplified by the closed-loop gain. When the external resistors that set the gain are changed by an amount corresponding to the digital step size, this value is added to the expected value, resulting in a differential nonlinearity error. If it is large enough, it can cause the DAC behavior to be nonmonotonic. To avoid this effect, it is necessary to choose an amplifier with low offset voltage and low input bias current. Advantages over other circuits In principle, if an external reference voltage source is allowed, a standard DAC can also be used, but there are some important differences between standard DACs and MDACs. The reference input of a standard DAC can only handle unipolar voltages of limited amplitude. In addition to the amplitude, the reference input bandwidth is also very limited. This is expressed in the data sheet as the multiplying bandwidth value. In the case of the AD5664 16-bit DAC, this value is 340 kHz. The reference input of a multiplying DAC can use bipolar voltages, which can also be higher than the supply voltage. The bandwidth is also much higher - the typical bandwidth of the AD5453 is 12 MHz. Conclusion Multiplying DACs are not so widely used, but they offer many possibilities. Besides high-bandwidth homemade PGAs, mobile applications are also very suitable applications because their power consumption requirements are less than 50 μW.