The THS413x is a fully-differential amplifier. Differential amplifiers are typically differential in/single out, whereas fully-differential amplifiers are differential in/differential out.
To understand the THS413x fully-differential amplifiers, the definition for the pin outs of the amplifier are provided.
If each output is measured independently, each output is one-half of the input signal when gain is 1. The following equations express the transfer function for each output:
The second output is equal and opposite in sign:
Fully-differential amplifiers may be viewed as two inverting amplifiers. In this case, the equation of an inverting amplifier holds true for gain calculations. One advantage of fully-differential amplifiers is that they offer twice as much dynamic range compared to single-ended amplifiers. For example, a 1-VPP ADC can only support an input signal of 1 VPP. If the output of the amplifier is 2 VPP, then it is not as practical to feed a 2-VPP signal into the targeted ADC. Using a fully-differential amplifier enables the user to break down the output into two 1-VPP signals with opposite signs and feed them into the differential input nodes of the ADC. In practice, the designer has been able to feed a 2-V peak-to-peak signal into a 1-V differential ADC with the help of a fully-differential amplifier. The final result indicates twice as much dynamic range. Figure 31 illustrates the increase in dynamic range. The gain factor should be considered in this scenario. The THS413x fully-differential amplifier offers an improved CMRR and PSRR due to its symmetrical input and output. Furthermore, second-harmonic distortion is improved. Second harmonics tend to cancel because of the symmetrical output.
Similar to the standard inverting amplifier configuration, input impedance of a fully-differential amplifier is selected by the input resistor, R(g). If input impedance is a constraint in design, the designer may choose to implement the differential amplifier as an instrumentation amplifier. This configuration improves the input impedance of the fully-differential amplifier. Figure 32 depicts the general format of instrumentation amplifiers.
The general transfer function for this circuit is:
Figure 33 and Figure 34 depict the differences between the operation of the THS413x fully-differential amplifier in two different modes. Fully-differential amplifiers can work with differential input or can be implemented as single in/differential out.
The power-down mode is used when power saving is required. The power-down terminal (PD) found on the THS413x is an active low terminal. If it is left as a no-connect terminal, the device always stays on due to an internal 50 kΩ resistor to VCC. The threshold voltage for this terminal is approximately 1.4 V above VCC–. This means that if the PD terminal is 1.4 V above VCC–, the device is active. If the PD terminal is less than 1.4 V above VCC–, the device is off. For example, if VCC– = –5 V, then the device is on when PD reaches –3.6 V, (–5 V + 1.4 V = –3.6 V). By the same calculation, the device is off below –3.6 V. It is recommended to pull the terminal to VCC– in order to turn the device off. Figure 35 shows the simplified version of the power-down circuit. While in the power-down state, the amplifier goes into a high-impedance state. The amplifier output impedance is typically greater than 1 MΩ in the power-down state.
Due to the similarity of the standard inverting amplifier configuration, the output impedance appears to be very low while in the power-down state. This is because the feedback resistor (Rf) and the gain resistor (R(g)) are still connected to the circuit. Therefore, a current path is allowed between the input of the amplifier and the output of the amplifier. An example of the closed loop output impedance is shown in Figure 36.