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AD8332ARU-REEL Datasheet(PDF) 17 Page - Analog Devices |
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AD8332ARU-REEL Datasheet(HTML) 17 Page - Analog Devices |
17 / 32 page AD8331/AD8332 Rev. C | Page 17 of 32 THEORY OF OPERATION OVERVIEW The following discussion applies to all part numbers. Figure 56 and Figure 1 are functional block diagrams of the AD8331 and AD8332, respectively. LNA 2 1 6 3 LMD INH COML VPSL 10 GAIN COMM LNA BIAS (VMID) VGA ∆G = –48dB to 0dB +21dB BIAS AND INTERPOLATOR 20 4 5 7 8 17 19 18 VMID POST AMP1 12 CLAMP GAIN INT 11 19 15 16 9 3.5dB/ 15.5dB ENBL ENBV RCLMP VOH VOL MODE LON LOP VIP VIN VPOS VCM 14 HILO COMM AD8331 Figure 56. Functional Block Diagram — AD8331 Each channel contains an LNA that provides user-adjustable input impedance termination, a differential X-AMP VGA, and a programmable gain postamplifier with adjustable output voltage limiting. Figure 57 shows a simplified block diagram. VOL VOH LNA CLAMP* – + INH LMD LOP LON HILO VCM PREAMPLIFIER 19dB X-AMP VGA POSTAMP [(–48 to 0) + 21] dB 3.5dB/15.5dB GAIN INTERFACE* GAIN BIAS AND INTERPOLATOR* VIN VIP *SHARED BETWEEN CHANNELS RCLMP BIAS (VMID) VMID Figure 57. Simplified Block Diagram The linear-in-dB gain control interface is trimmed for slope and absolute accuracy. The overall gain range is 48 dB, extending from –4.5 dB to +43.5 dB or from +7.5 dB to +55.5 dB, depending on the setting of the HILO pin. The slope of the gain control interface is 50 dB/V, and the gain control range is 40 mV to 1 V, leading to the following expressions for gain: ( ) () ( ) 1 , 5 6 50 ) ( LO HILO dB . – V V dB dB GAIN GAIN = × = or ( ) () ( ) 2 , 5 5 50 ) ( HI HILO dB . V V dB dB GAIN GAIN = + × = The gain characteristics are shown in Figure 58. 40 50 00.2 VGAIN (V) 0.6 0.4 30 10 0 20 –10 1.0 0.8 1.1 60 HILO = HI HILO = LO MODE = LO MODE = HI (WHERE AVAILABLE) Figure 58. Gain Control Characteristics When MODE is set high, (where available): ( ) () () 3 = , 5 45 + × 50 – = ) ( LO HILO dB . V V dB dB GAIN GAIN or ( ) () () 4 = , 5 57 + × 50 – = ) ( HI HILO dB . V V dB dB GAIN GAIN The LNA converts a single-ended input to a differential output with a voltage gain of 19 dB. When only one output is used, the gain is 13 dB. The inverting output is used for active input impedance termination. Each of the LNA outputs is capacitively coupled to a VGA input. The VGA consists of an attenuator with a range of 48 dB followed by an amplifier with 21 dB of gain, for a net gain range of –27 dB to +21 dB. The X-AMP gain-interpolation technique results in low gain error and uniform bandwidth, and differential signal paths minimize distortion. The final stage is a logic programmable amplifier with gains of 3.5 dB or 15.5 dB. The LO and HI gain modes are optimized for 12-bit and 10-bit A/D converter applications, in terms of output-referred noise and absolute gain range. Output voltage limiting may be programmed by the user. LOW NOISE AMPLIFIER (LNA) Good noise performance relies on a proprietary ultralow noise preamplifier at the beginning of the signal chain, which minimizes the noise contribution in the following VGA. Active impedance control optimizes noise performance for applications that benefit from input matching. |
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