Resistor networks are a great way to improve the gain accuracy and gain temperature drift of your system compared to designs using discrete resistors. One common motivation for improving gain accuracy and drift is that the initial system accuracy is good enough to skip calibration altogether. A common drawback of resistor networks is that there are a limited number of gain or attenuation options available, unlike discrete designs that can accommodate an essentially unlimited number of gain and attenuations.
This article shows how it’s possible to achieve 340 different gains and attenuations from just 10 resistor network ratios.
Take the case of RES11A, a resistor network that contains two resistor-divider networks (RG/RIN) in each package. The device is available in 10 product variants for different resistor ratios, with each variant identified by a suffix. For example, RES11A90 is the 9-to-1 ratio variant (see Figure 1).
You can use it to set gain on an operational amplifier or as an attenuator. The tolerance and drift of the resistors in the divider track closely, yielding significantly better attenuation accuracy and lower temperature drift than discrete resistors at a comparable price. RES11A, for example, has a worst-case tolerance of ±0.05% and temperature drift of ±2ppm/°C.
Figure 1 The above diagram highlights the functional blocks of the RES11A90 resistor network. Source: Texas Instruments
Swapping the input and output provides two different attenuations
The RES11A resistor network has 10 different resistor ratios, so you might think that you can only get 10 different attenuation values for this device. Keep in mind, however, that it’s not possible to swap the input and output of the attenuator, so each network can achieve at least two different attenuations.
Figure 2 shows how RES11A90 operates first as a 0.1 V/V and then a 0.9 V/V attenuator by simply reversing the connections. Thus, you can get at least 20 different attenuations from the 10 unique networks.
Figure 2 Swapping inputs and outputs on RES11A90 yields two different attenuations. Source: Texas Instruments
Combining both halves unlocks even more options
Generally, only one of the resistor pairs is active at a time when using RES11A as either an attenuator or a single-ended amplifier feedback network. But you can place the unused pair in series or parallel with either RIN or RG. Since all the resistors in the network were deposited in the same wafer processing step, they all have good ratio matching and drift matching.
Thus, the ratio, and drift accuracy of the combined divider will typically be the same as the individual dividers, although the worst-case ratio accuracy widens to ±0.1% with a ratio drift of ±4ppm/°C. Figure 3 illustrates how placing RG2 in parallel with RG1 achieves an attenuation of 0.818 V/V.
Figure 3 Using both halves of RES11A90 helps achieve a unique attenuation. Source: Texas Instruments
10 resistor network ratios, 34 configurations each
Different series and parallel combinations of both halves yield 34 different attenuations. Since there are 10 different variants of the network, the total attenuations possible with RES11A is 340 (34 × 10 = 340). Even accounting for duplicates, there are many unique attenuations. Figure 4 shows all 34 possible configurations.
Figure 4 Here are 34 possible attenuator configurations for the RES11A resistor network. Source: Texas Instruments
Manually sorting through so many options to determine which attenuator configuration meets your requirements isn’t practical. TI’s free Analog Engineer’s Calculator software tool identifies the best RES11A configuration to achieve your target attenuation or gain. Figure 5 illustrates how the calculator can find the resistor configuration for a target attenuation of 0.818 V/V.
Figure 5 The tool determines the RES11A configuration to achieve a 0.818 V/V attenuation. Source: Texas Instruments
Gain applications and compatible devices
While this article focuses on attenuation, RES11A also works well for setting gain on a single-ended or differential amplifier. In the case of a single-ended amplifier, combining both network divider halves results in many different gain values. Figure 6 shows the RES11A gain tool in the Analog Engineer’s Calculator to find different attenuations and gains.
Figure 6 This is how the tool finds gain of 3.5 V/V.
The tool also supports the RES21A and RES31A resistor networks, which share the same ratios as RES11A but scale overall resistance by a factor of 10 and 100, respectively. Thus, you can address your gain or attenuation requirement for RES11A and just substitute RES21A or RES31A if you require a higher overall resistance.
Ten ratio variants. Thirty-four configurations each. Three hundred forty reasons to leave the discrete resistor drawer closed.
Art Kay is applications engineer at Texas Instruments.
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