8.3.1 Rail-to-Rail Input Operation

The TLV237x input stage consists of two differential transistor pairs, NMOS and PMOS, that operate together to achieve rail-to-rail input operation. The transition point between these two pairs can be seen in Figure 2, Figure 3, and Figure 4 for a 2.7-V, 5-V, and 15-V supply. As the common-mode input voltage approaches the positive supply rail, the input pair switches from the PMOS differential pair to the NMOS differential pair. This transition occurs approximately 1.35 V from the positive rail and results in a change in offset voltage due to different device characteristics between the NMOS and PMOS pairs. If the input signal to the device is large enough to swing between both rails, this transition results in a reduction in common-mode rejection ratio (CMRR). If the input signal does not swing between both rails, it is best to bias the signal in the region where only one input pair is active. This is the region inFigure 2 through Figure 4 where the offset voltage varies slightly across the input range and optimal CMRR can be achieved. This has the greatest impact when operating from a 2.7-V supply voltage.

8.3.2 Driving a Capacitive Load

When the amplifier is configured in this manner, capacitive loading directly on the output decreases the device phase margin leading to high frequency ringing or oscillations. Therefore, for capacitive loads of greater than 10 pF, TI recommends that a resistor be placed in series (RNULL) with the output of the amplifier, as shown in Figure 34. A minimum value of 20 Ω should work well for most applications.

Figure 34. Driving a Capacitive Load

8.3.3 Offset Voltage

The output offset voltage, (VOO) is the sum of the input offset voltage (VIO) and both input bias currents (IIB) times the corresponding gains. Figure 35 can be used to calculate the output offset voltage:

Figure 35. Output Offset Voltage Model

8.3.4 General Configurations

When receiving low-level signals, limiting the bandwidth of the incoming signals into the system is often required. The simplest way to accomplish this is to place an RC filter at the noninverting terminal of the amplifier (see Figure 36).

Figure 36. Single-Pole Low-Pass Filter

If even more attenuation is needed, a multiple pole filter is required. The Sallen-Key filter can be used for this task. For best results, the amplifier must have a bandwidth that is 8 to 10 times the filter frequency bandwidth. Failure to do this can result in phase shift of the amplifier.

Figure 37. 2-Pole Low-Pass Sallen-Key Filter

8.3.5 Shutdown Function

Three members of the TLV237x family (TLV2370, TLV2373, and TLV2375) have a shutdown terminal for conserving battery life in portable applications. When the shutdown terminal is tied low, the supply current is reduced to 25 μA/channel, the amplifier is disabled, and the outputs are placed in a high impedance mode. To enable the amplifier, the shutdown terminal can either be left floating or pulled high. When the shutdown terminal is left floating, take care to ensure that parasitic leakage current at the shutdown terminal does not inadvertently place the operational amplifier into shutdown.