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AD640 DC-Coupled Demodulating 120 MHz Logarithmic Amplifier

Absolute calibration of a wideband logarithmic amplifier is unique. The AD640 is a high accuracy measurement device, not simply a logarithmic building block. Advanced design results in unprecedented stability over the full military temperature range. The fully differential signal path greatly reduces the risk of instability due to inadequate power supply decoupling and shared ground connections, a serious problem with commonly used unbalanced designs. Differential interfaces also ensure that the appropriate ground connection can be chosen for each signal port. They further increase versatility and simplify applications. The signal input impedance is ~500 kΩ in shunt with ~2 pF. The dc-coupled signal path eliminates the need for numerous interstage coupling capacitors and simplifies logarithmic conversion of subsonic signals. The low input offset voltage of 50 µV (200 µV max) ensures good accuracy for low level dc inputs. Thermal recovery “tails,” which can obscure the response when a small signal immediately follows a high level input, have been minimized by special attention to design details. The noise spectral density of 2 nV/√Hz results in a noise floor of ~23 µV rms (–80 dBm) at a bandwidth of 100 MHz. The dynamic range using cascaded AD640s can be extended to 95 dB by the inclusion of a simple filter between the two devices.

The AD640 is a complete monolithic logarithmic amplifier. A single AD640 provides up to 50 dB of dynamic range for frequencies from dc to 120 MHz. Two AD640s in cascade can provide up to 95 dB of dynamic range at reduced bandwidth. The AD640 uses a successive detection scheme to provide an output current proportional to the logarithm of the input voltage. It is laser calibrated to close tolerances and maintains high accuracy over the full military temperature range using supply voltages from ±4.5 V to ±7.5 V.

The AD640 comprises five cascaded dc-coupled amplifier/limiter stages, each having a small signal voltage gain of 10 dB and a –3 dB bandwidth of 350 MHz. Each stage has an associated full-wave detector, whose output current depends on the absolute value of its input voltage. The five outputs are summed to provide the video output (when low-pass filtered) scaled at 1 mA per decade (50 µA per dB). On chip resistors can be used to convert this output current to a voltage with several convenient slope options. A balanced signal output at +50 dB (referred to input) is provided to operate AD640s in cascade.

The logarithmic response is absolutely calibrated to within ±1 dB for dc or square wave inputs from ±0.75 mV to ±200 mV, with an intercept (logarithmic offset) at 1 mV dc. An integral X10 attenuator provides an alternative input range of ±7.5 mV to ±2 V dc. Scaling is also guaranteed for sinusoidal inputs.

The AD640B is specified for the industrial temperature range of –40°C to +85°C and the AD640T, available processed to MILSTD-883B, for the military range of –55°C to +125°C. Both are available in 20-lead side-brazed ceramic DIPs or leadless chip carriers (LCC). The AD640J is specified for the commercial temperature range of 0°C to +70°C, and is available in both 20-lead plastic DIP (N) and PLCC (P) packages. This device is now available to Standard Military Drawing (DESC) number 5962-9095501MRA and 5962-9095501M2A.

Product Highlights

• Absolute calibration of a wideband logarithmic amplifier is unique. The AD640 is a high accuracy measurement device, not simply a logarithmic building block.
• Advanced design results in unprecedented stability over the full military temperature range.
• The fully differential signal path greatly reduces the risk of instability due to inadequate power supply decoupling and shared ground connections, a serious problem with commonly used unbalanced designs.
• Differential interfaces also ensure that the appropriate ground connection can be chosen for each signal port. They further increase versatility and simplify applications. The signal input impedance is ~500 kΩ in shunt with ~2 pF.
• The dc-coupled signal path eliminates the need for numerous interstage coupling capacitors and simplifies logarithmic conversion of subsonic signals.
• The low input offset voltage of 50 µV (200 µV max) ensures good accuracy for low level dc inputs.
• Thermal recovery “tails,” which can obscure the response when a small signal immediately follows a high level input, have been minimized by special attention to design details.
• The noise spectral density of 2 nV/√Hz results in a noise floor of ~23 µV rms (–80 dBm) at a bandwidth of 100 MHz. The dynamic range using cascaded AD640s can be extended to 95 dB by the inclusion of a simple filter between the two devices.

Applications

• Radar, sonar, ultrasonic and audio systems
• Precision instrumentation from DC to 120 MHz
• Power measurement with absolute calibration
• Wide range high accuracy signal compression
• Alternative to discrete and hybrid IF strips
• Replaces several discrete log amp ICs

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Reference Materials

• Informational：ADI Warns Against Misuse of COTS Integrated Circuits
• Product Selection Guide：Space Qualified Parts List
• Product Selection Guide：RF Source Booklet
• Technical Articles：Measurement and Control of RF Power, Part I
• Technical Articles：Measurement and Control of RF Power, Part II
• Technical Articles：Measurement and Control of RF Power, Part III
• Technical Articles：Log Amps and Directional Couplers Enable VSWR Detection
• Technical Articles：Measuring VSWR and Gain in Wireless Systems
• Technical Articles：Measuring the RF Power in CDMA2000 and W-CDMA High Power Amplifiers (HPAs)
• Technical Articles：Detecting Fast RF Bursts using Log Amps
• Technical Articles：Make Precise Base-Station Power Measurements
• Technical Articles：Design a Logamp RF Pulse Detector