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AD8224 Datasheet(PDF) 24 Page - Analog Devices |
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AD8224 Datasheet(HTML) 24 Page - Analog Devices |
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24 / 27 page  AD8224 Preliminary Technical Data APPLICATIONS DRIVING AN ANALOG-TO-DIGITAL CONVERTER +IN store and deliver necessary charge to the switched capacitor input of the ADC. The 500 Ω series resistor reduces the burden of the + – AD8224 AD8224 REF2 An instrumentation amplifier is often used in front of an analog-to- RG +OUT +IN2 digital converter to provide CMRR and additional conditioning 20kΩ –IN such as a voltage level shift and gain (see Figure 63). In this example, a 2.7 nF capacitor and a 500 Ω resistor create an anti 33pF aliasing filter for the AD7685. The 2.7 nF capacitor also serves to 2.7 nF load from the amplifier. However, large source impedance in –OUT front of the ADC can degrade total harmonic distortion (THD). Figure 64. Differential Circuit Schematic For applications where THD performance is critical, the series Setting the Common-Mode Voltage resistor needs to be small. At worst, a small series resistor can load The output common-mode voltage is set by the average of +IN2 the AD8224, potentially causing the output to overshoot or ring. and REF2. The transfer function is In such cases, a buffer amplifier, such as the AD8615, should be used after the AD8224 to drive the ADC. VCM_OUT = (V+OUT + V−OUT)/2 = (V+IN2 + VREF2)/2 +5V +IN2 and REF2 have different properties that allow the AD8224 AD7685 ADR435 +5V 2.7nF REF 500Ω 1.07kΩ +2.5V +IN –IN 0.1µF 10µF + reference voltage to be easily set for a wide variety of applications. +IN2 has high impedance but cannot swing to the supply rails 4.7µF of the part. REF2 must be driven with a low impedance, but can go 300 mV beyond the supply rails. ±50mV A common application sets the common-mode output voltage to the midscale of a differential ADC. In this case, the ADC reference voltage is sent to the +IN2 terminal, and ground is connected to the REF2 terminal. This produces a common- mode output voltage of half the ADC reference voltage. Figure 63. Driving an ADC in a Low Frequency Application 2-Channel Differential Output Using a Dual Op Amp Another differential output topology is shown in Figure 65. Instead of a second in-amp, ½ of a dual OP2177 op amp creates the inverted output. Because the OP2177 comes in an MSOP, this configuration allows the creation of a dual channel, precision differential output in-amp with little board area. Errors from the op amp are common to both outputs and are thus common mode. Errors from mismatched resistors also create a common-mode dc offset. Because these errors are common mode, they are likely to be rejected by the next device in the signal chain. +IN AD8224 4.99kΩ + – OP2177 4.99kΩ +OUT REF VREF DIFFERENTIAL OUTPUT The differential configuration of the AD8224 has the same excellent dc precision specifications as the single-ended output configuration and is recommended for applications in the frequency range of dc to 100 kHz. The circuit configuration, outlined in Table 7, refers to the configuration shown in Figure 64 only. The circuit includes an RC filter that maintains the stability of the loop. The transfer function for the differential output is: VDIFF_OUT = V+OUT − V−OUT = (V+IN − V−IN) × G where: 49.4 kΩ G = 1 + –IN RG –OUT Figure 65. Differential Output Using Op Amp Rev. PrB | Page 24 of 27 |
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