To make the circuit "quiet", you need a buffer

In addition, the output voltage of the buffer is insensitive to the load resistance because the output resistance of an ideal buffer is essentially zero. Loading effects can be easily addressed by placing a unity-gain buffer between the digital-to-analog converter (DAC) and the load.

When adding a unity-gain buffer to a system, it is important not to sacrifice accuracy and performance. The most important point is to calculate the added noise:

in:

e _n = buffer input voltage noise density i _n = buffer input current noise density f = device input bandwidth (Hz)

In the circuit of Figure 1, the current noise of each channel is very low (0.8 fA/√Hz); in contrast, the voltage noise is (13 nV/√Hz). Therefore, to reduce the noise added to the system, this voltage noise must be reduced. Placing multiple buffers in parallel can reduce the voltage noise. For example, two buffers in parallel can reduce the voltage noise by √22, and all four buffers in parallel are equivalent to a buffer with half the noise. The disadvantage of this approach is increased bias current, current noise, and input capacitance, but in this case, these effects are negligible. Placing a small resistor such as 50 Ω between the outputs prevents additional current from flowing due to small differences between the outputs. For applications that are not sensitive to power consumption, these 50 Ω resistors can be omitted to increase the available output current.

Figure 1. The new AD8244 low-noise buffer.

The circuit shown in Figure 1 is a new configuration for a buffer amplifier that cuts voltage noise in half. The AD8244 is a quad, JFET-input, unity-gain buffer that offers much higher performance than expected. Its maximum bias current is 2 pA, current noise is near zero, and its input impedance is 10 TΩ, so even MΩ source impedances produce virtually no error. The device features low voltage noise, a wide supply range, and high precision, and is flexible enough to provide high performance for any application that requires a unity-gain buffer, even with low source resistance.

Figure 2 compares the noise performance of a conventional single-channel buffer and the new AD8244 buffer with four parallel channels.

Figure 2. Performance of the new AD8244 buffer: half the noise of conventional buffers.