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TPA2031D1 Datenblatt(PDF) 10 Page - Texas Instruments |
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TPA2031D1 Datenblatt(HTML) 10 Page - Texas Instruments |
10 / 24 page APPLICATION INFORMATION FULLY DIFFERENTIAL AMPLIFIER Advantages of Fully Differential Amplifiers COMPONENT SELECTION Input Resistors (RI) I 2 × 150 k V Gain = R V W æ ö ç ÷ è ø (1) TPA2031D1 SLOS546 – MAY 2008........................................................................................................................................................................................................ www.ti.com The TPA2031D1 is a fully differential amplifier with differential inputs and outputs. The fully differential amplifier consists of a differential amplifier and a common-mode amplifier. The differential amplifier ensures that the amplifier outputs a differential voltage on the output that is equal to the differential input times the gain. The common-mode feedback ensures that the common-mode voltage at the output is biased around VDD/2 regardless of the common-mode voltage at the input. The fully differential TPA2031D1 can still be used with a single-ended input; however, the TPA2031D1 should be used with differential inputs when in a noisy environment, like a wireless handset, to ensure maximum noise rejection. • Input-coupling capacitors not required: – The fully differential amplifier allows the inputs to be biased at voltage other than mid-supply voltage. For example, if a codec has a mid-supply voltage lower than the mid-supply voltage of the TPA2031D1, the common-mode feedback circuit adjusts, and the TPA2031D1 outputs are still biased at mid-supply voltage of the TPA2031D1. The inputs of the TPA2031D1 can be biased from 0.5 V to VDD –0.8 V. If the inputs are biased outside of that range, input-coupling capacitors are required. • Mid-supply bypass capacitor, C (BYPASS), not required: – The fully differential amplifier does not require a bypass capacitor. This is because any shift in the midsupply affects both positive and negative channels equally and cancels at the differential output. • Better RF-immunity: – GSM handsets save power by turning on and shutting off the RF transmitter at a rate of 217 Hz. The transmitted signal is picked-up on input and output traces. The fully differential amplifier cancels the signal much better than the typical audio amplifier. Figure 28 shows the TPA2031D1 typical schematic with differential inputs and Figure 29 shows the TPA2031D1 with differential inputs and input capacitors, and Figure 30 shows the TPA2031D1 with single-ended inputs. Differential inputs should be used whenever possible because the single-ended inputs are much more susceptible to noise. Table 1. Typical Component Values REF DES VALUE EIA SIZE MANUFACTURER PART NUMBER RI 150 k Ω (±0.5%) 0402 Panasonic ERJ2RHD154V CS 1 µF (+22%, –80%) 0402 Murata GRP155F50J105Z CI (1) 3.3 nF (±10%) 0201 Murata GRP033B10J332K (1) CI is only needed for single-ended input, or if VICM is not between 0.5 V and VDD–0.8 V. CI = 3.3 nF (with RI= 150 kΩ) gives a high-pass corner frequency of 321 Hz. The input resistors (RI) set the gain of the amplifier according to Equation 1. Resistor matching is very important in fully differential amplifiers. The balance of the output on the reference voltage depends on matched ratios of the resistors. CMRR, PSRR, and cancellation of the second harmonic distortion diminish if resistor mismatch occurs. Therefore, it is recommended to use 1% tolerance resistors or better to keep the performance optimized. Matching is more important than overall tolerance. Resistor arrays with 1% matching can be used with a tolerance greater than 1%. Place the input resistors very close to the TPA2031D1 to limit noise injection on the high-impedance nodes. 10 Submit Documentation Feedback Copyright © 2008, Texas Instruments Incorporated Product Folder Link(s): TPA2031D1 |
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