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Gain-of-two instrumentation amplifier uses no external resistors

An instrumentation amplifier offers precise gain without feedback resistors, and, at any value of gain, it provides high input impedances at its noninverting and inverting inputs. In a typical IC instrumentation amplifier, a single resistor that connects across two gain-adjustment pins determines the circuit's overall gain. Integrated versions of most instrumentation amplifiers allow the pins to remain open for unity gain but require finite-value gain-settingresistors for gains exceeding one.

Although the gain-adjustment resistor might comprise a tiny surfacemounted device, its electrodes and internal resistive layer extend the conductive surface connected to the IC's gain-adjustment pins. The extended surface acts as an antenna and thus makes the amplifier more susceptible to stray external electromagnetic fields.

Figure 1 shows an instrumentation amplifier that offers a gain of two without using any external resistors. The circuit comprises a cascade of a symmetrical, differential-output amplifier, formed by two channels of IC1; an Analog Devices (www.analog. com) AD8222 instrumentation amplifier; and a difference amplifier comprising one half of IC2, a second AD8222. All three instrumentation- amplifier sections in the that of the difference of the input signals.

The circuit's worst-case gain error does not exceed the value of
23
1, where, at a gain of one,
1 represents the maximum gain error of one section of the AD8222. For B-grade ICs, you calculate the value of
2 as
2 0.06% (Reference 1). Typically, the value of
2 rarely reaches its maximum value. Given the reasonable assumptions that all three amplifiers' gain errors are independent and obey a gaussian distribution, the probability of occurrence of
23
1 is about 1⁄20 the probability of encountering a single amplifier that has a maximum gain error of
1.

REFERENCE"AD8222 Precision, Dual-Channel Instrumentation Amplifier,"Analog Devices, www.analog.com/en/prod/0,2877,AD8222,00.html.

Analog switch converts 555 timer into pulse-width modulator

Jordan Dimitrov, Tradeport Electronics, Vaughan, Ontario, Canada

This Design Idea describes a new approach to producing a variable-duty-cycle waveform from a 555-based free-running oscillator. The circuit's wide modulation range, highly linear control over a wide range of duty-cycle values, and excellent linearity make it ideal for PWM (pulsewidth- modulation)-based control applications. Figure 1 shows the basic circuit, which works as follows: When IC1's output goes high, switch S1 closes, and IC1's internal discharge, switch S2, opens. Capacitor C1 charges through R1 and R2. When IC1's output goes low, S1 opens, and S2 closes, discharging C1through R2 and R3.

The generic configuration works well for producing a fixed-value duty cycle. Figure 2 shows how to obtain a necting potentiometer R4 to the common junction of R1, R2, and R3. The output waveform's duty cycle, DTC, follows the equation: DTC(R1R2RVAR)/ (R12R2R3RPOT), where RPOT is the potentiometer's end-to-end resistance, and RVAR is the fraction of RPOT between the rotor and R1. As the equation shows, DTC depends linearly on RVAR. Switch S1 comprises one section of a 4066 CMOS quad bilateral SPST switch, IC2.

You can use the circuit in Figure 3 to evaluate duty-cycle linearity. A rotary switch and a tapped series string of 16-k resistors provide a 10-kHz signal with nine discrete, equally spaced duty-cycle values ranging from 2 to 98%. For accurate results, use a 51⁄2- digit multimeter to match the values of resistors R4 through R11 and a Tektronix 3012 oscilloscope or equivalent to gather DTC data.

Microsoft's (www.microsoft.com) Excel-spreadsheet software includes a linearity analysis that returns the following trend line for the dutycycle measurements: DTC0.7565 RVAR2.1548; R21. The value of 1 for R2 as Excel calculates shows that the transfer function is perfectly linear. Switch S1's on-resistance and particularly its leakage current slightly affect the DTC-versus-RVAR equation's slope and intercept, but the equation remains strictly linear.

Using only one of IC2's four switches eliminates leakage effects and crosstalk that would occur if other circuits used the remaining switches. In addition, using moderately low values for the resistor network further reduces leakage-current effects on circuit performance.