THS3215 Datasheet, PDF (36/71 Pages) Texas Instruments – THS3215 650-MHz, Differential to Single-Ended DAC Output Amplifier

THS3215
SBOS780A – MARCH 2016 – REVISED APRIL 2016
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Feature Description (continued)
In the flat region, the 1-µF capacitor reduces the midscale buffer output spot noise from approximately 55 nV/√Hz
to 4.4 nV/√Hz. If the noise below 100 Hz is unacceptable, either add a low-noise buffer to drive this input, or add
lower-value resistors externally to set up the midsupply bias. Also, consider the noise impact of any reference
voltage source driving the midscale buffer path.
9.3.3 Output Power Stage (OPS) (Pins 4, 7, 9, 10, 11, and 12)
A wideband CFA provides a flexible output driver with several unique features. The OPS can be left unused if the
specific application only uses the D2S alone, or a combination of the D2S with an off-chip power driver. If left
unused, simply tie DISABLE (pin 10) and PATHSEL (pin 4) to the positive supply. This logic configuration turns
the OPS off and opens up the external and internal OPS noninverting input paths. An internal fixed 18.5-kΩ
resistor holds the external input pin at the logic reference voltage on GND (pin 7). Additionally, the OPS output is
connected to the inverting input through another internal 18.5-kΩ resistor when no external resistors are installed
on VIN+, VOUT, or VIN– (pins 9, 11, or 12, repectively). Disabling the OPS saves approximately 11 mA of supply
current from the nominal total 35 mA, with all stages operating on ±6-V supplies.
The noninverting input to the OPS provides two possible paths controlled by the PATHSEL logic control. With the
logic reference (pin 7) at ground, floating PATHSEL or controlling it to a voltage < 0.7 V connects the input path
directly to the internal D2S output. Tying PATHSEL to the positive supply, or controlling it to a logic level > 1.3 V,
connects the input path to the external input at VIN+. The intent for this switched input is to allow an external
filter to be inserted between the D2S output and OPS inputs when needed, and bypass the filter when not.
Alternatively, this switched input also allows a completely different signal path to be inserted at the OPS input,
independent of that available at the internal D2S output.
In situations where the D2S output at VO1 (pin 6) is switched into another off-chip power driver, the OPS can be
disabled using DISABLE. With the logic reference (pin 7) at ground, floating DISABLE, or controlling it to a
voltage < 0.7 V, enables the OPS. Tying DISABLE to the positive supply, or controlling it to a logic level > 1.3 V,
disables the OPS.
Operation of the wideband, current-feedback OPS requires an external feedback resistor and a gain element.
After configuring, the OPS can amplify the D2S output through either the noninverting path, or be configured as
an inverting amplifier stage using the external OPS input at VIN+ as a dc reference.
One of the first considerations when designing with the OPS is determining the external resistor values as a
function of gain in order to hold the best ac performance. The loop gain (LG) of a CFA is set by the internal
open-loop transimpedance gain from the inverting error current to the output, and the effective feedback
impedance to the inverting input. The nominal internal open-loop transimpedance gain (Z_OL) magnitude and
phase are shown in Figure 82.
140
45
Phase
Magnitude
120
0
100
-45
80
-90
60
-135
40
1k
10k 100k
1M
10M 100M
Frequency (Hz)
-180
2G
D502
Figure 82. Simulated OPS Z_OL Gain Magnitude and Phase
36
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