Signal Chain Basics: Exploring Programmable Gain Amplifier Drive Reference Pins

Programmable gain amplifier (PGA) is a special amplifier configuration (see FIG. 1), having an internal trimmed resistor network, the amplifier has a higher performance than the discrete resistor assembly. As shown in the PGA transfer function in Figure 1 , the absolute error of the PGA output is related to the internal offset voltage (VOS), gain accuracy, and VREF absolute accuracy.
Figure 1 PGA configuration example of the corresponding transfer function

In some applications that use PGA, the key DC specifications are VOS, gain accuracy and offset, noise, and static power. If the reference pin, VREF, is not driven by an op amp buffer, the accuracy of the PGA transfer function is greatly affected. In addition, from the AC's point of view, a common problem is the gain accuracy at the maintenance frequency, which is affected by the reference pin voltage VREF and the op amp that buffers it.

Taking into account the bandwidth, AOL (ω), the case where the feedback coefficient ([omega]) of the operational amplifier and the buffer circuit (beta]) (see FIG. 2) the size of the RO, we can better understand the effect of the operational amplifier VREF generated influences.


Figure 2 Vref buffer divider voltage

Since the buffer itself is β = 1, the output voltage VREF is equal to AOLVIN. The input bias current that VREF flows into the inverting input of the buffer amplifier determines the magnitude of the load current. This is important because the magnitude of the load current regulates the loop gain (AOLβ) and the closed-loop output impedance ROUT.

Figure 2 shows the closed-loop internal circuitry of the VREF buffer: the important relationship between Rout, Ro, and AOL is shown in Equation 1 :
Equation 1

In summary, as frequency increases, op amps have the ability to maintain a fixed output voltage and low impedance by reducing AOL, increasing Rout, and extending settling time. This affects the accuracy of the PGA gain error.

For the sake of explanation, consider the example of the single-ended PGA shown in Figure 3 . The input signal VIN has its DC component (2.5V), while the AC signal is a 200 mVpp, 5 kHz sine wave:

Figure 3 buffer single-ended PGA


Figure 4 analyzes Figure 5 with the “ Multimeter ” function in TINA Spice .

We can use the TINA Spice "Multimeter" function (see FIG. 4), the RMS value of the input voltage of the output voltage and used to calculate the total output error, the specific calculation method as shown in Equation 2 and 3:

Equation 2

Equation 3

For example, the micropower precision op amp OPA333 has a gain bandwidth (GBW) of ~350 kHz. Therefore, at 5 kHz, the closed-loop characteristic drops to a level that causes a 0.08% error at the output of the second op amp (such as OPA376). This error can be reduced by using a higher GBW amplifier (eg another precision op amp).

By plotting the transfer function diagram (VOUT / VIN) in the frequency graph in TINA SPICE, we can visually see the effect of changing the impedance of the frequency (see FIG. 5). Note that the gain vs. frequency is more constant when the OPA376 acts as a buffer compared to the OPA333:

Figure 5 OPA333 and OPA376 buffer comparison chart

The results show that using a higher bandwidth op amp (eg OPA376, etc.) as a VREF buffer amplifier can significantly improve the overall output error.

Next time, we will discuss the increasing THD (cause and method analysis) in audio processing systems.


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