Impedance/current of 2180 control voltage input

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MaxDM

Well-known member
Joined
Oct 23, 2018
Messages
206
I see that on most application designs, the control input of the 2180 is being driven from the output of an op amp.

I was wondering if the control input could be driven from a higher source impedance, like a resistor divider, such as 10K/and a 1K pot, which would be the attenuator for compression ratio.

I looked on the datasheet, but found nothing about source impedance etc.
 
You must not have looked very hard. It's on page 9 of the pdf: http://www.thatcorp.com/datashts/THAT_2180-Series_Datasheet.pdf

Control Port Drive Impedance

The control ports (pins 2 through 4) are connected directly to the bases of the logging and/or antilogging transistors. The accuracy of the logging and antilogging is dependent on the EC+ and EC voltages being exactly as desired to control gain. The base current in the core transistors will follow the collector currents, of course. Since the collector currents are signal-related, the base currents are therefore also signal-related. Should the source impedance of the control voltage(s) be large, the signal-related base currents will cause signal-related voltages to appear at the control ports, which will interfere with precise logging and antilogging, in turn causing distortion.

The 2180 Series VCAs are designed to be operated with zero source impedance at pins 2 and 3, and an infinite source impedance at pin 4. (pin 4 should be left open.) To realize all the performance designed into a 2180, keep the source impedance of the control voltage driver well under 50 Ω.
 
right you are!

Ok, so it ideally needs less than 50 Ohm source.

thanks
 
right you are!

Ok, so it ideally needs less than 50 Ohm source.

thanks
+1 those control ports are bases of transistors so termination needs to be low impedance to prevent distortion from base current generating control error voltages from resistances there. Take care if driving directly from an op amp output, while surely low impedance any op amp noise injected into that control port will show up in the VCA signal output.

The THAT application notes include suggested circuits. Usually a good starting point.

JR
 
Further reading of the THAT datasheet talks about an RC network on the opamp output pin to counteract the inductive nature of the opamp's output:
This often suggests driving the control port directly with an opamp. However, the closed-loop output impedance of an opamp typically rises at high frequencies because open loop gain falls off as frequency increases. A typical opamp's output impedance is therefore inductive at high frequencies. Excessive inductance in the control port source impedance can cause the VCA to oscillate internally. In such cases, a 100 Ω resistor in series with a 1.5 nF capacitor from the control port to ground will usually suffice to prevent the instability.
It's a small addition but can make a huge improvement. And I've seen this in many THAT app notes too.
 

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