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Diff-Amp Uses a Resistor Network

Low-cost integrated resistor networks can be used to implement a low-parts-count two-op-amp differential amplifier with low common-mode gain. Integrated onto a common substrate and processed together, resistors of integrated networks match well and are an alternative solution to discrete resistors, especially for amplifiers with a wide common-mode input voltage range. The amplifier circuit is shown below.

In some diff-amp applications such as high-side current sensing, where the sense resistor voltage to ground can have a large range, it is important that the op-amp differential amplifier (diff-amp ) reject the floating voltage of the sense resistor. In other words, the common-mode (CM) gain, ACM of the diff-amp must be minimized. The above amplifier circuit is a two-op-amp diff-amp with half of an 8-R resistor network. Two parts (using a quad op-amp) can implement a dual diff-amp circuit with low error caused by CM gain.

Two sources for CM gain are the op-amps used in the diff-amp and the gain-setting resistors around the op-amps. Amplifier CM rejection is usually specified as the CM rejection ratio ,

where Av is the (desired) differential gain,

and vo = amplifier output voltage. The (undesirable) amplifier CM gain is

where the common-mode voltage is

and is the average of the voltages at the inputs. As the CM voltage changes, ideally no change in the output voltage of the diff-amp occurs, and only the difference voltage at the diff-amp inputs is amplified. For the above circuit with equal resistors in the network, the quasistatic gain is Av0 = 2.

The derivation of A¬CM caused by resistor mismatch is rather involved. (See Planet Analog articles, Seemingly Simple Circuits, Part 3: Effect of Resistor Tolerances on Diff-Amp Gains and Seemingly Simple Circuits, Part 4: Diff-Amp Common-Mode Rejection by this author for details.) However, it reduces to a rather simple approximate formula:

where ε = resistor tolerance. A +/-0.2 % resistor network matching tolerance has ε = 0.002.

Thick-film R-networks are low-cost and readily available from passive-components manufacturers. They typically are specified at 1 % to 2 % absolute inaccuracy. What matters for low A-CM is how well the resistors match, and being made in a common process on a common substrate, they match typically to 0.2 % to 0.4 %. Thick-film TCR tracking is typically about 50 ppm/o C, comparable to T2 1 % metal-film resistors.

Suppose that the resistors match to 0.33 %, or ε = 0.0033. Then the CM gain of the given diff-amp is

The CM error resolution is 6.83 bits and ACM ≈ 8.8 mV/V. At a +/-0.2 % match, the error resolves to 1/187.5 = 5.33 mV/V, or about 7.55 bits. For many applications, this CM error is acceptably low. If so, a CM trimpot adjustment (of the bottom R of the string) is eliminated from the circuit. If it is not sufficient but close, such as for an 8- to 10-bit system, then software calibration can remove the CM error. (See Part 2 of the above-mentioned diff-amp article series for that.) Greater precision can be achieved using thin-film R-networks at greater cost – an alternative to discrete +/-0.1 % metal-film resistors.

A 16-pin SMT or DIP R-network has 8 matched resistors, usually of the same value. For a gain of up to 4, the extra resistors can be placed in series or parallel to modify Av . Placing resistors of a given tolerance in either series or parallel results in the same tolerance for the combinations.

3 comments on “Diff-Amp Uses a Resistor Network

  1. sroochi
    April 28, 2016

    Interesting article!  As you are aware, this circuit was used by Analog Devices and National Semiconductor some ~30yrs ago.  Worked well then, and probably better today with today's better OP amps.

    I used this circuit when I designed my high side/low side gate driver input stage.  I needed fully differential performance to +/-1200V — using only 30V Vbdss devices with similar gate to substrate breakdowns.  It was a simple matter of using a pair of matched 1000:1 attenuation networks (..placed in front of the differential amplifier circuit) created with poly silicon resistors over thick field oxide — 2um thickness (thermal oxide has a breakdown characteristic of ~ 100V/.1um.)  

    All the Best,

    Sam Ochi

     

     

     

  2. traneus
    April 29, 2016

    sroochi's +/-1200-volt on-chip common-mode-range differential input: Excellent!

  3. sroochi
    May 5, 2016

    I have used this technique to create a product, IX6R11S3, a 6Amp high side/low side gate driver with +/-600V high side to low side gate drive swing capability about 10 yrs ago.  I was granted a patent, #6759692, “Gate Driver With Level Shift Circuit,” issued 10/23/03.  

    Enjoy,

    Sam Ochi

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