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AD6640 Datasheet(PDF) 11 Page - Analog Devices |
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AD6640 Datasheet(HTML) 11 Page - Analog Devices |
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11 / 24 page  AD6640 –11– REV. A THEORY OF OPERATION The AD6640 analog-to-digital converter (ADC) employs a two- stage subrange architecture. This design approach ensures 12-bit accuracy, without the need for laser trim, at low power. As shown in the functional block diagram, the AD6640 has complementary analog input pins, AIN and AIN. Each analog input is centered at 2.4 V and should swing ±0.5 V around this reference (see Figure 2). Since AIN and AIN are 180 degrees out of phase, the differential analog input signal is 2 V p-p. Both analog inputs are buffered prior to the first track-and-hold, TH1. The high state of the ENCODE pulse places TH1 in hold mode. The held value of TH1 is applied to the input of a 6-bit coarse ADC. The digital output of the coarse ADC drives a 6-bit DAC; the DAC is 12 bits accurate. The output of the 6-bit DAC is subtracted from the delayed analog signal at the input of TH3 to generate a residue signal. TH2 is used as an analog pipeline to null out the digital delay of the coarse ADC. The 6-bit coarse ADC word and 7-bit residue word are added together and corrected in the digital error correction logic to generate the output word. The result is a 12-bit parallel digital CMOS compatible word, coded as twos complement. APPLYING THE AD6640 Encoding the AD6640 A valid ENCODE clock must be present on the AD6640 before the application of AVCC (5 V). Best performance is obtained by driving the ENCODE pins differentially. However, the AD6640 is also designed to interface with TTL and CMOS logic families. The source used to drive the ENCODE pin(s) must be clean and free from jitter. Sources with excessive jitter will limit SNR (see the first equation under the Noise Floor and SNR section). 0.01 F TTL OR CMOS SOURCE ENCODE ENCODE AD6640 Figure 7. Single-Ended TTL /CMOS ENCODE The AD6640 ENCODE inputs are connected to a differential input stage (see Figure 3). With no input signal connected to either ENCODE pin, the voltage dividers bias the inputs to 1.6 V. For TTL or CMOS usage, the ENCODE source should be connected to ENCODE, Pin 3. ENCODE should be decoupled using a low inductance or microwave chip capacitor to ground. If a logic threshold other than the nominal 1.6 V is required, the following equations show how to use an external resistor, Rx, to raise or lower the trip point (see Figure 3; R1 = 17 k Ω and R2 = 8 k Ω). V l = 5R2Rx R1R2 + R1Rx + R2Rx to lower logic threshold. 0.01 F ENCODE SOURCE ENCODE ENCODE AD6640 Rx Vl 5V R1 R2 Figure 8. Lower Logic Threshold for ENCODE V R R RRx RRx l = + + 52 2 1 1 to raise logic threshold. 0.01 F ENCODE SOURCE ENCODE ENCODE AD6640 Rx Vl 5V R1 R2 AVCC Figure 9. Raise Logic Threshold for ENCODE While the single-ended ENCODE will work well for many appli- cations, driving the ENCODE differentially will provide increased performance. Depending on circuit layout and system noise, a 1 dB to 3 dB improvement in SNR can be realized. It is not recom- mended that differential TTL logic be used because most TTL families that support complementary outputs are not delay or slew rate matched. Instead, it is recommended that the ENCODE signal be ac-coupled into the ENCODE and ENCODE pins. The simplest option is shown below. The low jitter TTL signal is coupled with a limiting resistor, typically 100 Ω, to the primary side of an RF transformer (these transformers are inexpensive and readily available; part number in Figure 10 is from Mini- Circuits). The secondary side is connected to the ENCODE and ENCODE pins of the converter. Since both ENCODE inputs are self-biased, no additional components are required. TTL ENCODE ENCODE AD6640 100 T1–1T 0.1 F Figure 10. TTL Source–Differential ENCODE A clean sine wave may be substituted for a TTL clock. In this case, the matching network is shown. Select a transformer ratio to match source and load impedances. The input impedance of the AD6640 ENCODE is approximately 11 k Ω differentially. Therefore the “R,” shown in the Figure 11, may be any value that is convenient for available drive power. |
Similar Part No. - AD6640_15 |
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Similar Description - AD6640_15 |
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