Wide Ranging dB Meter THAT Corporation Design Note 119 wwwthatcorpcom infothatcorpcom Copyright ID: 217474
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complement our standalone VCAs. We encourage readers to consider the following alterna-Low supply voltage and power consumption: 4320Low cost, supply voltage, and power consumption: 4315Low cost and power consumption: 4305High-performance (VCA only): 2180-series, 2181-seriesDual (VCA only): 2162RMS (standalone): 2252 Wide Ranging dB Meter THAT Corporation Design Note 119 www.thatcorp.com info@thatcorp.com Copyright © 2000 - 2012 by THAT Corporation; All rights reserved.Document 600158 Revision 02 the THAT4301's RMS detector by compressing the VCA's 120dB dynamic range through theX is the VCA input, and we will let its full scale be 20 dBu.ks is the stand-by gain, independent of the control port, taken in the VCA. We willY is the VCA output, measured in dBu, and is also the RMS detector input.RL is the zero dB reference level of the RMS detector. This is the input level whichG is the current gain of the VCA, in dB, as controlled via its control port. It is also a,or (eqn. 3) Copyright © 2000-2012Page 2 of 6All rights reserved. 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.comDoc. 600158 Rev. 02 InputVCA Out(Y)RMS Out(G)12dBstandbygain (ks) Z e r o dB r e f e r ence level (RL) 20-(-100)=120dB, the dynamic range of the VCA output is only 34-(-26)=60dB, thusdemonstrating the 2:1 compressor action. The gain range of +34dB to -26dB is well within Copyright © 2000-2012Page 3 of 6All rights reserved. 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.comDoc. 600158 Rev. 02 -10-20-30-40-50-60-70-80-90-100-14-19-24-29-34-39-44-49-54-26-21-16-11VCA Input in dBuVCA Output in dBuVCA current gain in dB +3300ppm9k110u442kR1060k4R113M9R1220k-15 +15V100nR1480kC1247p-15V+15V0 dB Ref.TrimVR250kOA3EC+EC-SYMGNDOUTU1A4301POUTRMS4301POA1U1D4301P10u-15V51RInputOA2U1C4301POutputSPAREFS=20dBuLevel match=-20dBu3.25mV/dB-39.3 mV/dB12dB of standby gain100n Application Circui t This circuit consists of a VCA with 12dB of stand-by gain, an RMS detector connected to thedesigned to have a full scale input of 20dBu and an output that is scaled to transform aThis circuit accepts a full scale signal of 20 dBu. If the source is not that high, one canconfigure the spare op-amp, U1C, as an amplifier to scale the signal appropriately. In mostapplications, log-antilog VCAs are used with zero dB of stand-by gain; that is, when thebeneficial to use stand-by gain to skew the compressor's gain requirements. If not, then themaximum VCA gain required would be +40 dB when the VCA input dropped to -100 dBu, whichC12 helps OA3 maintain stability against the high output capacitance of the VCA. Thiscapacitor, in conjunction with R14, set the bandwidth at ~56 kHz. Since this is a measure-case distortion without trimming symmetry is about 0.7 % THD, and this will not result in a1.Converting the input into a current and full wave rectifying it.2.Logging and doubling this current waveform, which in the log domain, is equivalent3.Averaging the result in a log-domain filter, which results in the mean of that signal. Copyright © 2000-2012Page 4 of 6All rights reserved. 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.comDoc. 600158 Rev. 02 TimingIn this design, the timing current is set to the recommended value by R6:R6=T 7.5 Thus, =VTId , at 27ºCrd=VTId 3.499If we set the corner frequency to , =125 C1= 3.466 9.2ZEROdB9.6 9.6 ZEROdBAs stated earlier, we chose -20dBu as the zero dB reference level. This corresponds to:ZEROdB0.775 dBu 77.5ZEROdBZEROdB 77.58.5 9.1The gain control constant for the THAT4301 is 6.5mV/dB, but due to the 2:1 compressoraction, the change at the output of the RMS detector is only 3.25mV/dB. This voltage ismeter's output to within ±½dB, the RMS detector's output needs to be scaled tobits bits 0.039 Copyright © 2000-2012Page 5 of 6All rights reserved. 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.comDoc. 600158 Rev. 02 In this design, OA1 is configured as an inverting summing amplifier. R3 should be a+3300 ppm/ºC resistor, as this will correctly compensate the temperature coefficient ofthe level detector's gain control constant (the temperature coefficient of the VCA's gain 1203.25 12.864.9corresponds to 20 dBu, and -100 dBu is near 5V. If this becomes an issue, OA2 can be used0.00325 k 0.65 -100 dBu-to-20 dBu proportional to 5V-to-0V. As stated previously, the zero dB referencelevel point is at -20 dBu at the RMS, but there is also 12 dB of voltage gain in the VCA, andthe zero dB reference level relative to the VCA's input is -20 dBu -12 dB = -32 dBu. When 0.65 R11 provides ±6 dB of adjustment, to accommodate the ±3dB tolerance on the zero dB 0.65 3.9read with ½dB resolution by an inexpensive, 8-bit DAC. Copyright © 2000-2012Page 6 of 6All rights reserved. 45 Sumner St, Milford, MA 01757 USA; www.thatcorp.comDoc. 600158 Rev. 02