that4315 THAT Corporation, that4315 Datasheet - Page 4

• Manufacturer Part Number: that4315
• Description: Pre-trimmed Low-voltage Low-power Analog Engine
• Manufacturer: THAT Corporation

Page 4
This pin is a virtual ground with dc level approxi-
mately equal to V
put voltage is converted to input current via an
appropriately sized resistor. Because the currents
associated with dc offsets present at the input pin
and any dc offset in preceding stages will be modu-
lated by gain changes (thereby becoming audible as
thumps), the input pin is normally ac-coupled.
with respect to the input current. In normal opera-
tion, the output current is converted to a voltage via
external op-amp, where the ratio of the conversion is
determined by the feedback resistor connected be-
tween op-amp‘s output and its inverting input. The
signal path through the VCA and op-amp is
noninverting.
applied between EC+ and EC-. Note that any unused
control port should be connected to the Vref genera-
tor. The gain (in decibels) is proportional to (EC+ –
EC-). The constant of proportionality is 6.0 mV/dB
for the voltage at EC+ (relative to EC-), but note that
neither EC+ or EC- should vary more than ±0.5 V
from V
a predictable way, but due to the way internal bias
currents vary with gain, noise at the output is not
strictly the product of a static input noise times the
voltage gain commanded. At large attenuation, the
noise floor of ~ -109 dBV is limited by the input
noise of the output op-amp and its feedback resis-
tor. At 0 dB gain, the noise floor is ~ -98 dBV as
specified. In the vicinity of 0 dB gain, the noise in-
creases more slowly than the gain: approximately
5 dB noise increase for every 10 dB gain increase.
Finally, as gain approaches 30 dB, output noise be-
gins to increase directly with gain.
that of the THAT 2180 Series VCAs, there are sev-
eral important differences, as follows:
At +5 V V
the sum of input and output signal currents. This in-
creases to about 1 mA at +15 V V
to ~1.8 mA for a 2180 Series VCA when biased as
recommended. This is appropriate given the lower
supply voltage for the 4315.)
2180 VCA) exists, but is driven from an internally
trimmed current generator.
Input signals are currents in the VCA’s IN pin.
The VCA output signal is also a current, inverted
The gain of the VCA is controlled by the voltage
The VCA’s noise performance varies with gain in
While the 4315’s VCA circuitry is very similar to
1. Supply current for the VCA depends on V
2. A SYM control port (similar to that on the
REF
.
CC
, approximately 500 mA is available for
REF
THAT Corporation; 45 Sumner Street; Milford, Massachusetts 01757-1656; USA
, so in normal operation an in-
Tel: [phone]; Fax: [phone]; Web: www.thatcorp.com
CC
. (Compare this
Copyright © 2009, THAT Corporation
CC
.
6.0 mV/dB, due primarily to the lower internal oper-
ating temperature of the 4315 compared to that of
the 2180 Series (and the 4301).
the 4315 uses less emitter degeneration resistance
in its output than that of the 2180 VCA. This re-
quires that the source impedance at the VCA’s input
(which is a summing junction) must be under 5 kW
at frequencies over 1 MHz. See the applications sec-
tion for an alternative on how to address this issue.
The RMS Detector — in Brief
tifying input current signals, converting the rectified
current to a logarithmic voltage, and applying that
voltage to a log-domain filter. The output signal is a
dc voltage proportional to the decibel-level of the
RMS value of the input signal current. Some ac com-
ponent (at twice the input frequency) remains super-
imposed on the dc output. The ac signal is
attenuated by a log-domain filter, which constitutes a
single-pole rolloff with cutoff determined by an ex-
ternal capacitor.
RMS
level equal to V
normally used to convert input voltages to the de-
sired current. The level detector is capable of accu-
rately resolving signals well below 10 mV (with a
5 kW input resistor). However, if the detector is to
accurately track such low-level signals, ac coupling
is required. Note also that small, low-voltage elec-
trolytic capacitors used for this purpose may create
significant leakage if they support half the supply
voltage, as is the case when the source is
dc-referenced to ground. To ensure good detector
tracking to low levels, a high quality electrolytic may
be required for input coupling.
placed well below the frequency range of interest.
For an audio-band detector, a typical value would be
5 Hz, or a 32 ms time constant (t). The filter’s time
constant is determined by an external timing capaci-
tor attached to the CT pin, and an internal current
source (I
internally fixed at 7.5 mA. The resulting time con-
stant in seconds is approximately equal to 3467
times the value of the timing capacitor. Note that, as
a result of the mathematics of RMS detection, the at-
tack and release time constants are fixed in their re-
lationship to each other.
3. The control-voltage constant is approximately
4. The OTA used for the VCA’s internal opamp in
The 4315’s detector computes RMS level by rec-
As in the VCA, input signals are currents to the
The log-domain filter cutoff frequency is usually
IN
pin. This input is a virtual ground with dc
T
) connected to CT. The current source is
REF
, so a resistor, R24 in Figure 2, is
THAT4315 Pre-trimmed Low-voltage
Low-power Analog Engine® IC