AD600/AD602
THEORY OF OPERATION
The AD600 and AD602 have the same general design and fea- tures. They comprise two fixed gain amplifiers, each preceded by a voltage-controlled attenuator of 0 dB to 42.14 dB with in- dependent control interfaces, each having a scaling factor of
32 dB per volt. The gain of each amplifier in the AD600 is laser trimmed to 41.07 dB (X113), thus providing a control range of –1.07 dB to 41.07 dB (0 dB to 40 dB with overlap), while the AD602 amplifiers have a gain of 31.07 dB (X35.8) and provide an overall gain of –11.07 dB to 31.07 dB (–10 dB to 30 dB with overlap).
The advantage of this topology is that the amplifier can use negative feedback to increase the accuracy of its gain; also, since the amplifier never has to handle large signals at its input, the distortion can be very low. A further feature of this approach is that the small-signal gain and phase response, and thus the pulse response, are essentially independent of gain.
The following discussion describes the AD600. Figure 1 is a simplified schematic of one channel. The input attenuator is a seven-section R-2R ladder network, using untrimmed resistors of nominally R = 62.5 Ω, which results in a characteristic resis- tance of 125 Ω ± 20%. A shunt resistor is included at the input and laser trimmed to establish a more exact input resistance of 100 Ω ± 2%, which ensures accurate operation (gain and HP corner frequency) when used in conjunction with external resis- tors or capacitors.
| | | GAT1 | |
| SCALING | PRECISION PASSIVE | GATING | |
| INPUT ATTENUATOR | |
| INTERFACE | |
REFERENCE | |
| | |
C1HI | | | | A1OP |
VG | | | |
C1LO | | | | |
GAIN CONTROL | | | |
INTERFACE | | RF2 | |
| | | 2.24kΩ (AD600) | |
| | | 694Ω (AD602) | |
0dB | –6.02dB –12.04dB –18.06dB | –22.08dB –30.1dB –36.12dB –42.14dB | RF1 | |
A1HI | | | 20Ω | |
| | | |
500Ω | 62.5Ω | FIXED GAIN | A1CM |
|
A1LO | | | AMPLIFIER | |
| R – 2R LADDER NETWORK | 41.07dB (AD600) | |
| | | 31.07dB (AD602) | |
Figure 1. Simplified Block Diagram of Single Channel of the AD600 and AD602
The nominal maximum signal at input A1HI is 1 V rms (± 1.4 V peak) when using the recommended ± 5 V supplies, although operation to ± 2 V peak is permissible with some increase in HF distortion and feedthrough. Each attenuator is provided with a separate signal “LO” connection, for use in rejecting common- mode, the voltage between input and output grounds. Circuitry is included to provide rejection of up to ± 100 mV.
The signal applied at the input of the ladder network is attenu- ated by 6.02 dB by each section; thus, the attenuation to each of the taps is progressively 0, 6.02, 12.04, 18.06, 24.08, 30.1, 36.12 and 42.14 dB. A unique circuit technique is employed to inter- polate between these tap points, indicated by the “slider” in Fig- ure 1, providing continuous attenuation from 0 dB to 42.14 dB.
It will help, in understanding the AD600, to think in terms of a mechanical means for moving this slider from left to right; in fact, it is voltage controlled. The details of the control interface are discussed later. Note that the gain is at all times exactly de- termined, and a linear decibel relationship is automatically guar- anteed between the gain and the control parameter which determines the position of the slider. In practice, the gain devi- ates from the ideal law, by about ± 0.2 dB peak (see, for ex- ample, Figure 6).
Note that the signal inputs are not fully differential: A1LO and A1CM (for CH1) and A2LO and A2CM (for CH2) provide separate access to the input and output grounds. This recog- nizes the practical fact that even when using a ground plane, small differences will arise in the voltages at these nodes. It is important that A1LO and A2LO be connected directly to the input ground(s); significant impedance in these connections will reduce the gain accuracy. A1CM and A2CM should be con- nected to the load ground(s).
Noise Performance
An important reason for using this approach is the superior noise performance that can be achieved. The nominal resistance seen at the inner tap points of the attenuator is 41.7 Ω (one third of 125 Ω), which exhibits a Johnson noise spectral density (NSD) of 0.84 nV/√Hz (that is, √4kTR) at 27°C, which is a large fraction of the total input noise. The first stage of the am- plifier contributes a further 1.12 nV/√Hz, for a total input noise of 1.4 nV/√Hz.
The noise at the 0 dB tap depends on whether the input is short-circuited or open-circuited: when shorted, the minimum NSD of 1.12 nV/√Hz is achieved; when open, the resistance of
100Ω at the first tap generates 1.29 nV/√Hz, so the noise in- creases to a total of 1.71 nV/√Hz. (This last calculation would be important if the AD600 were preceded, for example, by a
900Ω resistor to allow operation from inputs up to ± 10 V rms. However, in most cases the low impedance of the source will limit the maximum noise resistance.)
It will be apparent from the foregoing that it is essential to use a low resistance in the design of the ladder network to achieve low noise. In some applications this may be inconvenient, requiring the use of an external buffer or preamplifier. However, very few amplifiers combine the needed low noise with low distortion at maximum input levels, and the power consumption needed to achieve this performance is fundamentally required to be quite high (due to the need to maintain very low resistance values while also coping with large inputs). On the other hand, there is little value in providing a buffer with high input impedance, since the usual reason for this—the minimization of loading of a high resistance source—is not compatible with low noise.
Apart from the small variations just discussed, the signal-to- noise (S/ N) ratio at the output is essentially independent of the attenuator setting, since the maximum undistorted output is 1 V rms and the NSD at the output of the AD600 is fixed at 113 times 1.4 nV/√Hz, or 158 nV/√Hz. Thus, in a 1 MHz band- width, the output S/N ratio would be 76 dB. The input NSD of the AD600 and AD602 are the same, but because of the 10 dB lower gain in the AD602’s fixed amplifier, its output S/N ratio is 10 dB better, or 86 dB in a 1 MHz bandwidth.
